Her surprise at his quick acceptance. I felt the burn in my chest... He didn't even put up a fight. And I get it... Tom though no saint himself suffered throughout this marriage. And even in this conversation, how she wouldn't let him talk, still kinda invalidating his feelings...
I got the feeling throughout this whole sequence that Shiv did not really buy everything she was saying. But Tom had lost what little fight he had left in him so he just accepted it.