This is my speculation through observation, but imho friendship in its pure form is a genetic program. If you're "no longer twenty these days" (semi-quote from Bernie Taupin), you may not remember how it was but just look at any teen company in public transport or elsewhere, it just feels like a single organism. And at that age it doesn't mean whether your views (if there are any) are similar, you're just attracted. Another speculation that this mutation could be tossed several times with different outcomes, like different spans of this attraction. And keeping the strong bonds later in life from the evolution standpoint it wasn't worth it really so the best fit was to narrow it to some period best for survival of both participant (or group)