Tim Kreider said this quote

“ When you're a child, your best friend in the world is the kid who lives next door. It doesn't occur to you then that this is a matter of arbitrary circumstance. When you grow up you like to imagine that your friendships have a more substantial basis - common interests, like-mindedness, some genuine affinity. It's always a sad revelation that when a good friend acquires a girlfriend or a husband and disappears. You realize that,for them, your friendships was always only a matter of convenience, a fallback, and they simply don't need you anymore. There's nothing especially cynical about this; people are drawn to each other because they're giving each other something they both need, and they drift apart when they aren't getting it or don't need it anymore. Friendship have natural life spans, like love affairs or favorite songs. ”