In chassidus, we learned about the prototypical friendship: Reuven understands that by hanging out with Shimon he will gain something (witty words of wisdom, free Starbucks coffee, whatever) and therefore Reuven starts to love Shimon and spend time with him. (The nimshal being that through hisbonenus, one understands how great G-d is and starts to love him. Do not try this at home. It will not work. ואכ"מ)
That's not how my friendships formed. I became friends with people when external circumstances threw us together, even though we weren't especially compatible, and even though we didn't work on maintaining the friendship. Many of my closest friends were roommates. We never chose to become friends. We ended up in the same room by coincidence. But we ended up spending lots of time together and just getting to know each other. So we became friends.
Sorry folks, but I'm going to need to bring another example from the Chassidic masters. The Frierdiker Rebbe says that there are two kinds of mercy: mercy from exaltedness ("romemus") and mercy from empathy ("hergesh," literally "feeling"). The first kind of mercy is what the king feels towards the beggar in the street. Something along the lines of, "I'm the big king. I have a palace and hordes of servants at my beck and call. You have nothing. All you have is a spot on the sidewalk near the subway's air shaft to keep you warm in the winter. I will mercifully give you fifty cents."
The Frierdiker Rebbe does not define mercy from empathy at much length (as far as I can recall), but this is what I think he means: I and my friend spend time together. We share with each other. There becomes some kind of connection, and the line between "I" and "you" starts to blur. He loses five dollars, and parts of his frustration leaks into me. His brother gets engaged, and I experience part of the joy. Why do I care if my roommate's brother gets engaged any more that if some random other guy gets engaged? Will it make a difference to my life at all? It doesn't make a difference. But it makes a difference to my friend. And what makes a difference to him makes a difference to me.
Many years ago my sister got a book out from the library about a pair of Siamese twins. They shared a stomach, so that when one of them drank, they both got drunk. That's friendship. Your alcohol affects my stomach, because we share parts of ourselves with each other.
So don't ask me why I care. I care because I feel for you. Because you're my friend. Maybe it was some silly set of circumstances that brought us together. That's irrelevant. "vi nisht vi" the circumstances were what they were, and we came together. So now we're together, and we share whatever experiences life sends our way.