It's not what he expected. Not the kiss, that he kind of saw coming, but the gentleness of it and damn if it doesn't undo him completely. Underneath it all he's still a man, building walls with machismo and ruthlessness but a man nonetheless and he's so tired. Everything he's gone through in the past century doesn't measure up to what a few measly months here has done to him and it's the loneliness that gets him, blinding, relentless, more so now that he can't go back to the flat and have someone there.
It's empty and he's never going back.
So he doesn't pull back, not immediately, just lets it happen and when he does break the kiss it's not to punch or snarl or anything. Instead he leans his head on Jack's shoulder, forehead pressed heavily against the other man as if the weight of being alive is just about to crush him. And then a protest, like an echo.