Depression is a very difficult state of mind.  When an individual is in a low mood, it can affect almost every aspect of his or her life - behavior, feelings, thoughts, and even well-being.  Do you know anyone who battles with this?
I do actually, i know many who struggle with depression personally. It is a truly devastating mental illness. 
I agree.  It runs in my wife's family.  Obviously, things like the loss of a loved one can trigger it; but there are so many causes, including physical, hormonal, or mental issues that can trigger depression.  Have those friends of your been diagnosed with clinical depression?
They have yes. The causes however were due to hormonal imbalances and they are being treated with CBT and SSRI's.
What is so difficult in many of these cases is that some will have periods of depression separated by even years in which they are normal, but then years of depression again.  It is such a difficult state to be in, and so many in life battle with this in silence.  People will foolishly tell them just to get over it, and sometimes, many times, people can't.
Yes, i am very well aware of this. I am actually a psychology graduate student and have seen this in many textbook cases. Relapse is very frequent and can be caused as you said by a number of factors. It's simply an inescapable illness.
Clinical psychology
Clinical psychology is an integration of science, theory, and clinical knowledge for the purpose of understanding, preventing, and relieving psychologically-based distress or dysfunction and to promote subjective well-being and personal development. Central to its practice are psychological assessment, clinical formulation, and psychotherapy, although clinical psychologists also engage in research, teaching, consultation, forensic testimony, and program development and administration. In many countries, clinical psychology is a regulated mental health profession.
The field is generally considered to have begun
Depression (mood)
Depression is a state of low mood and aversion to activity. It can affect a person's thoughts, behavior, motivation, feelings, and sense of well-being. It may feature sadness, difficulty in thinking and concentration and a significant increase/decrease in appetite and time spent sleeping, and people experiencing depression may have feelings of dejection, hopelessness and, sometimes, suicidal thoughts. It can either be short term or long term. Depressed mood is a symptom of some mood disorders such as