It’s just how he is. Ianto has come to have a better understanding of it over the past few years, enough to accept that even without Spike in the picture Jack never would have stayed with him forever. He isn’t built for it. It was a sad realization at first, but he tries not to think about it much these days.

He should have been expecting that kiss but it still takes him by surprise so he fumbles for a moment before returning it, soft and emotional but perhaps not quite as needy as Jack. That hesitation isn’t something he’s really conscious of. Perhaps it’s a bit of reluctance, a slight feeling that they still aren’t quite there yet. It’s clear however that Jack needs this and that tugs at him, too, makes him press back against those lips even if he isn’t sure this is the best move for himself just now.