"There was a lot of political fallout from the end of the second war and I'd lost the ability to do magic, which made it both miserable and rather dangerous for me to remain in London. I chose the name Sebastian and built up a new identity as a muggle, but before that, -- what I mean is, the reason I know so much about Harry's life, is that it was my own." It's not a smooth revelation and he doesn't know if she'll accept it, but he couldn't go about his life in the city without having told her.