Don't lie you love it as much as I do
He has no intention of trying to get back together again for the exact same reason. He gave Jack chance after chance after chance over the years, made excuse after excuse and he's tired of it. At first he told himself that he wasn't in a position to say anything, not when he was the one who started all this by cheating on Spike. When that stoped convincing him he told himself it was inevitable, that he always knew this was how Jack was, that it was no different from the way things were back at Torchwood. But eventually that excuse stopped working too and he had to ask himself why that should mean anything. They'd both changed since then, their relationship had changed and there was only so long he could lie to himself before admitting the truth; if Jack didn't value this enough to stay with him then it wasn't worth all the pain and annoyance anymore. They said a lot of things in that lasg fight that ended everything but he's not sure he ever really explained all of that clearly in the heat of the moment and he can feel it all simmering under the surface.
Maybe that's what prompts his acid, passive-aggressive tone as he can't contain himself anymore and asks, "Had a good time last night?" One downside of those sensitive vampire ears is knowing exactly what Jack gets up to and who with. Kirk was a friend right from the begining and he can't help but wonder if Jack's taken up with him specifically because he knows it will hurt him.