Each new thing he learned about Henry's life (and undeath) made Peter feel more sympathy for him. Getting turned into a vampire was bad enough, but losing his wife, and then whoever Abraham was, presumably a best friend, was enough to break someone.

"I don't know if I'd want to live forever," he says. "But I understand. You still empathize with others. You want them to have fulfilling lives." Observing that could be painful, but important for closure.