As teens and young adults start to figure out their identity and interests, they seek out other people with those shared interests. The place today to reliably find people with your niche interests (if you don't live in a major metro area) is social media.
As you age, your social graph starts to ossify. Unless you take deliberate effort to put yourself in generative spaces where you meet a lot of new people, you'll tend to want to spend time with the people you already know.