Honestly I think one of the underappreciated aspects of tomgreg is that... I don't think Tom has had a real friend before - or if he has, it was a very long time ago. He's in this world where everything is false and everything has a veneer of artifice and everyone keeps themselves and their emotions under lock and key, and everyone is at everyone else's throats. And he's been in that world probably since college, when he started trying to climb the social ladder, and the problem is that you can't really have actual friendships in that world. You can have people who you do things with and people that you talk to regularly, but you can't have the kinds of emotional intimacy that really characterize a friendship.
And on top of that, Tom is... off-putting, and he knows it. He comes on too strong and he can't quite master the mannerisms and jokes and air of being unaffected that are requisite for fitting into this world. I really strongly get the sense that he felt like an outsider even before he started dating Shiv and became part of the Roy's sphere of influence for this exact reason.
And I think he really does feel, especially from 1.06 onwards, that he has an actual friend in Greg. The way that he and Greg gravitate towards each other in the background of every group scene? The way that Tom rushes to tell him about the bj from Tabitha at his bachelor party? Boar on the floor? "Just fucking come over"? We can go on and on about the power imbalance and Tom's need to feel dominant over somebody and the emotional needs that aren't being met in his marriage, and all of that is true, but I think it's also so clear that he really does value Greg as a person and as a friend and as someone he can talk to and demonstrate some degree of emotional vulnerability with. Greg's not intimidating and Greg lets Tom's emotional outbursts roll off him and Greg doesn't shun or judge Tom for his offputting mannerisms, because he's also a little offputting and he knows it.
(And while I think Greg's feelings towards Tom are a little more complicated than Tom's feelings towards Greg, I think he really does consider them friends and value the friendship too. First of all bc he's actually explicitly referred to Tom as his friend, but just as Tom seems more comfortable around Greg than he does around anyone else, so does Greg seem to be comfortable around Tom).
you’re in a panic room with a beautiful boy and he won’t tell you that he wants to work in another department, but he wants to work in another department and you feel like he’s breaking up with you so you start throwing water bottles at him