[He looks surprised by her surprise--at least until she says the reasonable things about timing and chemistry. Just because he suspects that the problem is him doesn't mean that he can't see her rationale.] It would be irresponsible to attribute everything to circumstances and chemistry.

[Maybe it should be comforting to know that, if the circumstances and chemistry are right, even terrible people can do alright for themselves? So, inversely, okay people can fail utterly multiple times through no real fault of their own? ...That is a lot to unpack and he chooses not to address it directly.]

I'm not certain that I will. The times that I thought that maybe I had... Tessa left and fell in love with two other people. I was with Lucy for seventeen months and she still went back to a home where she had nothing and no one. [No need to mention Kitty, she knows that one.] No one who is only unlucky with timing and chemistry can spend at least twenty-six years in four different universes without finding a single person who wants them.