If Tommy had accepted Quackity's help and stayed with him, I think he would have grown to view Quackity as a role model/brother figure similar to how hd viewed Wilbur at the beginning. So he imitates, even if only subconsciously, certain habits.
Like drinking a whole bottle of wine (no more than one) very quickly after a bad or stressful day on a mostly empty stomach and just enjoying the buzz running through his veins and lying in bed the rest of the day with that buzz.
I wonder if Quackity walks in on Tommy copying one of his bad habits, what would he do. Yes, duh, he would feel bad and guilty and blame himself, but he wouldn't address his emotions if it could save him. I think in an attempt to ignore and justify his own habits, he'd coo at Tommy and card his fingers through blond hair and hold his head while it's spinning and smile down at him and put hix head in Quackity's lap.
Maybe kisses. Forehead first and for a while. Quackity is deluding himself that he doesn't want to kiss Tommy on the lips. He wants to, of course, so badly, but it seems wrong.
Until maybe, one time they do this coping mechanism at the same time and Quackity or Tommy misses the door to their own room (they have rooms right next to each other) and walk in on each other. They'd cuddle first, all drunkenly protective of each other. Then one rubs their face against the other's and that triggers a full on buzzed make out session. All sloppy and spit everywhere but their heads are spinning pleasantly and thus feel right, the pressure, the wetness, the warmth, the connection.
Of course it happens more than once. Of course they get worse. But it's not misery spent alone, and that alone is a balm to the soul they would lay the world to waste for.
