How is the theme of regret depicted within Kafuku's reflections on his late wife and their life together?

Kafuku regrets deeply that he had never confronted his wife about her infidelities before her death. He still does not understand what in the world would have made his wife want to do such a thing. The theme of regret is deeply embedded in this part of the story, as Kafuku has no way to rectify his psychologically troubling situation. He wants to know why his darling wife had felt the need to sleep with other men, but since she is gone, he will never be able to hear her thoughts on the matter. And still he is plagued by thoughts of his late wife in the arms of other men.