Tony likes Bruce, he does, because he gets it. He understands the pain he goes through, having to fight off your other half, being afraid to care about someone. Fighting off monsters when you yourself are a monster? It's the most fucked up thing to deal with, and Tony, he's just...Tony. There isn't some switch that'll flip when he's angry, make him not himself, no no no. The monster in Tony is actually Tony. That sick twisted part of him that wants to die, wants to be left alone and at the same time wants to hurt everyone and everything because of his past. Because of how damaged he is.
And Bruce? Bruce makes him feel better, makes him happy in some screwed up way because he's finally not the only one who hates themself. Makes it easier to breathe when he can say a few words to him about that part of him and he won't be judged, he'll be understood. He has a friend for once, he knows he doesn't have many, Pepper and Rhodey are and he thought Obie was but- He can't deal with that anymmore, too much pain, the darkness swells up in him a lot, because of him, he trusted him. And look what he did.
He used to drown his problems in alcohol, he still does, though not as much, because he has Bruce. He can talk to him, work in the lab with him, share blue prints and joke around, all with that underlying current of rightness, of belonging. He can't get attached, but he is, because he knows himself, one day he'll do or say something that'll make Bruce leave. and he's an idiot for getting attachted, he knows this, knows what normally happens because of him, but he can't help it.
He needs him.