What I like is that it can go both ways. You can be completely solitary and just do your own thing, and that's fine.

But your friend is completely right that there can also be a lot of impromptu taking turns on problems, sharing beta, cheering each other on, showing off, etc..

I think I'm often more extroverted when bouldering than in any other context in my life (geek with Asperger's, etc.). Family and friends are completely boggled by the fact that when bouldering, I actually spontaneously chat to strangers.