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From your wording in your post it seemed like you thought all depression was a consequence on social issues, and myself and presumably others rejected that idea. 
The important thing is that there is no one qualifier, and by saying depressions core issue is sociological it may put up a barrier on entry for those who suffer deppression with no social problems. Because they would not meet that qualification of "all depression is caused by social issues" 
Thats what I came here to argue against, but it seems you donrt hold the that veiw I had thought.
I think if you reword your thought and focus more on the symptoms of the person and less of  the causes people will be  far more agreeable
I come from an upper-middle class family, both parents still married and happy after over 25 years, always had a roof over my head, food in my stomach, and an excess of material items beyond my needs. I have no severe traumas, never had a TBI, never been abused, and so forth. By all definitions, I should be happy without any issues, and I usually am.
My father struggles with depression. We both can understand what it’s like to feel like a literal cloud rolls in from beyond the horizon and over our heads, and like a light switch, something in our brain makes a change and it starts to rain. We are both diagnosed with major depressive disorder, so is my paternal grandfather. We all three take the same antidepressant, Wellbutrin, and have for years. The brother of my 3rd great-grandmother (so my 3rd great-uncle) shot himself in the head and killed himself when he was in his early-30s, back in the late 1800s. Depression runs in my family, there’s no question.
Both my dad and I have a mutation in our SLC6A4 gene, which is responsible for producing serotonin receptors in the brain, that causes us to produce less receptors than the average person. There are several studies that suggest a connection between this mutation and depressive disorders and OCD. The link is still not certain, though. Nonetheless I think it’s an important thing to mention. There is ongoing research to understand how genetics play into depression.
My biggest trigger for a depressive episode is stress. When I experience enough of it in a certain period of time, my brain’s response is to essentially shut down. Look up the diathesis-stress model. It theorizes that psychological disorders are a result of the interaction between predispositional vulnerabilities (genetics, trauma, etc.) and stress from life experiences. This is why while my brother or even someone unrelated to me won’t develop depression in response to X amount of stress, others like myself will.
This is a biological thing, it is real and it is observable. Depression is an incredibly complex and multi-faceted issue, and it’s not always purely genetic. But as you can see for me, I have at least two direct, and one distant relative, that experiences/experienced depression. >

Thanks so much for sharing your experience and some detailed relevant facts! I will go have a think about this. Comments like this are definitely making me think. >

I’m glad you were able to read it. I can be pretty all over the place when it comes to topics I’m passionate about, so I’m happy it went to use.
Here’s my big point: yes, the world can be depressing, but that doesn’t mean we all have depression. Yes, we all likely find things depressing, but that doesn’t debilitate the entire population as a result.
Say a loved one dies, you get sad and that’s normal. It’s a sad thing to happen and the normal response is to feel sad about it. Experiencing depression is when it falls outside that normal range of sadness, where it’s been weeks or months since the loss and you still can’t get out of bed, eat a proper meal, or leave the house. Worse yet, people dealing with depression might not need to lose a loved one to experience those things, they might lose their job, experience some sort of hardship like divorce or a car accident or other trauma, and they can’t continue to function in their daily life.
Feeling sad about sad things is normal and healthy to an extent, it’s when it consumes your very being and everything you do that it starts to resemble clinical depression. >

Depression can be purely biological, purely emotional, a mixture of the two, or the first coming as a result of long-standing emotional.  
Our brains and mood run on neurotransmitters, and there can be a genetic predisposition to be lacking in those neurotransmitters, but also since our brains work on feedback loops, it is also possible to get into a feedback loop that is difficult to get out of without the help of medication.  
Of course, you can also have situational depression, where you are legitimately sad for valid reasons.  Sometimes antidepressants can help prevent that from becoming a feedback loop.  
For example, there's biological depression - everything in life is going great, but someone don't feel great and is depressed, despite having no legitimate reason to be depressed. Therapy is going to have some benefit there, to have coping mechanisms to deal with it, but antidepressants are going to be the main treatment, because there's nothing "wrong" in your life. 
Situational depression happens when a loved one dies or a relationship you value is lost, and a person feels depressed. It's absolutely valid as depression, and therapy has a huge amount of benefit there, because it is a specific issue that you need to learn to deal/cope with. Antidepressants can help sometimes, because they help with the neurotransmitters, which are suppressed because of your mood, but therapy is the best help.
You can also have situational superimposed onto biological depression - you have depression that is controlled with medication, but something bad has happened in your life that makes it worse.  Therapy would help on top of medication adjustment in that case.  
In other words, depression is complex, multifactorial and can change through the course of the disease. >


Situational depression happens when a loved one dies or a relationship you value is lost, and a person feels depressed.

This isn't depression though and even if you probably know that, just be careful saying it without clarifying. It's perfectly normal to be depressed when a loved one dies. The problem is when that depressed state persists. People who are grieving will often have short periods of time where they are very sad and then they will also have times where they feel ok, that is a healthy response. A person who is depressed as a result of the death of a loved one will be in a depressed state for a more continuous time. That's the thing to look out for. Not am I low mood because of my grief but does it persist more than it should.  
Also to add even more complexity if everything in life is going great but you feel depressed it may be your thought patterns and behaviors that are reinforcing the way you feel. So therapy can still be very helpful and may be better than medication. I think that's why the combination is pretty consistently shown to have the best outcomes. >


This isn't depression though and even if you probably know that, just be careful saying it without clarifying

I'm under the impression that any period of depression that lasts longer than 14 days is clinical depression. Our medical system doesn't differentiate based on the reason for the depression. >


Situational depression happens when a loved one dies or a relationship you value is lost, and a person feels depressed

You are correct but that's not what was stated. Also that's 14 days of depression not 14 days where during some short periods you felt depressed.
to be more clear here is some diagnostic criteria. To be depressed you would show 5 of the following with at least 1 of them being the first two over a 14 day period:

Depressed mood most of the day, nearly every day.
Markedly diminished interest or pleasure in all, or almost all, activities most of the day, nearly every day.
Significant weight loss when not dieting or weight gain, or decrease or increase in appetite nearly every day.
A slowing down of thought and a reduction of physical movement (observable by others, not merely subjective feelings of restlessness or being slowed down).
Fatigue or loss of energy nearly every day.
Feelings of worthlessness or excessive or inappropriate guilt nearly every day.
Diminished ability to think or concentrate, or indecisiveness, nearly every day.
Recurrent thoughts of death, recurrent suicidal ideation without a specific plan, or a suicide attempt or a specific plan for committing suicide. >

I didn't specify a time frame, and was meaning to imply it would be longer standing. But there are absolutely situations that can cause you to be depressed, clinically, if you do not have the coping skills to handle it >

I just wanted to clarify because some people mistake regular grief and being in a depressed state for depression. It did seem like you probably knew that and were referring to when people sink into a depression from their grief but I just wanted to clarify for people reading. >

So it's important we use the correct language here. 
Sad, depressed, and depression disorder, are different things. 
Being sad is an emotion. Being depressed or suffering depression are overarching feelings. Having depression disorded is a biological thing. 
The way I like to think about it is in weather terms. Being sad is like having a day where it rains. Being depressed is like there is a monsoon or hurricane. The rain doesn't last just a few hours/ a day, but multiple days. Depression disorder is like the rainforest. It rains every day and and getting sunny weather is very difficult because the trees block it out. 
Furthermore, just because someone is feeling depressed but is not diagnosed with depression disorder does not make their depression any less valid. Depression disorder however is a real biological problem. It is a chemical imbalence. 
You can also develop this chemical imbalence, so if youre depresed but not crossing the line to full blown depression disorder, you can develop that imbalence and get full blow depression disorder. 
For psychologists and doctors this disticntion is important . But for the general public it doesnt matter nearly at all and we should try to treat everyone with as much love and respect as possible at all times, regardless if they are just sad, depressed or have the disorder. >

Very true. I have experienced both kinds of “depression”, and multiple doctors agree. What you said about how someone can develop that chemical imbalance is important, though. I have decided that genetic predispositions and physical abnormalities are definitely mechanisms involved in depression, which is a wholly physical phenomenon - but I’m not convinced that the core issue isn’t sociological. I came up with a random analogy in a response to another comment about an imaginary society that lifts heavy objects all day, in which some people have a predisposition to weaker bones. >

Sure, if people in that fake society didn't lift rocks people with weak bones wouldnt be at as much as a disadvantage. 
There is definetly some merit to what you are saying, but to paint with a braod brush and say all depression is socialogical I don't agree with. Maybe a lot but definetly not all. 
Depression isn't always about being just sad, but it can be lethargy or not being able to be happy. Like not being sad but just not being able to acheive the  highs of life if that makes sense. 
There might be nothing sociological going on, no peer pressure, no social issues, no romantic issues. The person might just not feel happy. 
But all these issues regarding mental health all go hand in hand. There is no problem on the earth that is simple enough to not have 1000 aterisks attached to it and interactions with hundreds of other factors. 
So while I disagree to the notion the core issue is sociological, I can agree that that might be one of the largest contributing factors. >

I think I actually agree with this entire comment. Some of my previous wording has been misleading. What is written after your last comma is the crucial position I hold that most people here seem to reject.