Life is relationship and relationship is life, let's accept that.
So what exactly is relationship? Is it something which you know each other like he is my brother, he is my father, she is my wife, she is my mother.
Relationship is relating. If you relate to someone on the same level then you are in a relationship but what is this same level? This same level is when you drop all your ambitions when you drop all your greed, when you drop all your fears and you are really compassionate towards the other person you are really relating to that person.
I think that relationship starts with caring and caring can only take place when you have nothing to ask for yourself from that person in exchange. (But ironically caring can give you a illusion of control over the other person, a sense of possessiveness which ultimately might become abuse in the relationship).
It is often said that parents damage their children more than anyone else, a classic case of caring turning into abuse.