I mean in that you see them now and again, share a meal and know a bit about their lives. I have no interest in being his best friend 
I'm trying very hard not to mind but I do.
20yo DS1 lives at home but has a very serious GF so is rarely here, except on the nights she stays over, when they get in late, go to bed and stay there until I've gone out in the morning.
In six months I've only met her a handful of times to have an actual conversation with. She's nice and she's making him happy, it's all good.
However, they spend loads of time at her house, his parents are lively characters and he's enjoying being part of this big lively family. Her father tagged him in some FB photos last night. They were playing party games after dinner, something he wouldn't want to join in with if I suggested it.
There's only me and practically reclusive DS2 at home. DH died this year. I get it's not as much fun, I understand why they'd prefer to be elsewhere, but this is really hurting me.
I know I can't set up a "competition" between us, I'd lose and rightly so, but what can I do to avoid losing my son completely?