Growing up, your choice of friends is usually based around proximity and interests. For all its faults, the public education system has created an extremely effective method for crowding adolescents together and forcing them to pick sides. Unless you're one of those 12-year-old musical prodigies or Daniel Radcliffe, you really have nothing to separate you from the pack of kids whose greatest achievement is passing the grade prior to the one they're in now. You're left with nothing but your interests, whether they be sports or video games or video games about sports, the last one rendering you permanently insufferable.
College tends to divide people based on who they want to be. The people who want to light up Broadway are paired with other future waiters. The people who hold up pictures of dead fetuses in the middle of campus will catch the eyes of other abortion protestors in the Elementary Education building, and sparks will fly. The people who want to get drunk after their business exams will inevitably seek like-minded souls, and together they'll throw a couch off of my friend Tim's porch.
Though, to be fair, that's when the house stopped smelling weird.
Post-college looks at the wreckage and piles people into groups based on what they're doing. I was living at home, so my pickings were getting slimmer and slimmer. The place I was an intern at was filled with older people who had actual jobs and spouses and children, so they had no desire to join a desperate me for drinks after 5. I had remained friends with a scant few that I'd known in high school, but most of them had left for other parts of the state and country, and I hung out with them only when they visited town and felt enough sympathy to yell my name three times into the cave.