I doubt it is the psychology, but the lack of shared experiences outside work. In school you have shared experiences and you grow to like some of the guys you hang out with. You don't hang out (much) at work and so don't have the shared experiences and consequently make no bonds.

i'd say dwingling neuroplasticity and brain chemistry.

Actually it's just harder to meet new people. And harder still to meet interesting new people. (Especially ones who aren't judgemental about age.)