“It was a fixed point. It was… it, it needed to happen. Somethings, some events, history revolves around them. I can’t change that.”

That really works for me as a reason why he wasn't there. When Gwen was saying the he "turns from this planet in shame" in my head I was muttering "no, he doesn't" (which did make me feel I might be taking it a bit too seriously *g*) as nothing I've seen in my years of watching Doctor Who would make me believe that.

Excellent last line. Not many people could have made Jack's choice.