Friendship

Is it weird to anyone else how we pick friends? We see someone, like something about them, begin a conversation, and from there it's a friendship. But my question is why do friendships end? I mean to some the answer is simple, people move away, people grow up, people just drift away. But what happens when you think that you're doing everything right. You've changed nothing in the friendship, yet it isn't working for you like it was before. I don't know I just think it's a lot harder for me to stop caring so quickly about a friendship, than it is for the other person. But people change and times do so as well.