What the reason was -

[He checks himself, for once. She's not from his universe, she's still alive, and she had a daughter named Thea. There was no guarantee the boating accident had even happened.]

Boating accident. I got stranded on an island in the Fijis for a few years, so I'm sorry if I didn't make it in time for senior year.

[He's clearly not sorry at all.]

You're treating me like a child.