Should I always put another person's interest before mine? I try to become a kinder person. Sometimes I feel bad ( guilty) after not doing a service for someone, even if it's not in my benefit to do that thing. How can I understand when it's kindness toward another and when it's "stepping over me"?
In Buddhism, there are fundamental realizations that help you answer these kinds of questions via direct experience, especially the realization of non-self.

There is in fact no "other" that does stepping on.
There is in fact no "me" that is being stepped on.

You are not just yourself. "You" are the whole universe (in a sense).

Anyway, when you can realize that then you will be able to let go enough to stop feeling sorry for yourself and be able to see things objectively more clearly.

That is what the following advice is based on.

If people did not pay for services and people just gave them away then two things would happen:


Those people would in fact not value the service/gift that they are receiving
The person doing the giving will go hungry and perish, one way or another


Furthermore, this delicate exchange that I am referring to, if disrespected by letting other people "step on you" by not getting something back in value (whether it is the practice itself be it teaching or whatever service you are offering), then they will eventually form a debt with you and a chronicity of theievery in their own mind.

In Buddhist tradition, monks don't just beg and run off after they get a meal. They tend to share dharma with their benefactor.

Something to think about: by letting other people "step on you", you are enabling them to do an evil deed! Do not let them use you! This evil deed is not only their seed of retribution but your own to experience in the future as well.

So be strong, be assertive. Make sure you are happy with the exchange! Give and take value. Even if that value is just the action itself, make sure you are happy with it before doing it and not doing it too much out of guilt or pity. If you have to do it out of pity, then try to assert that you are just "making one exception" so that there is no chronicity in the person's mind (which strangely enough is your mind as well!)

I am sure I lost you somewhere and I would need to talk much longer in order to clarify this deep, deep matter but the following things will help you solidify and strengthen your thinking in this regard as you live in this world.


Read MindOS 2nd edition (the best psychology book that exists)
Read Diamond Sutra Explained
Do Metta meditation
Meditate on the feeling of being "stepped on"... find out where it is coming from... talk to it in a journal. Ask this feeling what will make it feel better and happy. You will have surprising insights when you can open up to.. yourself.