The fun thing is that it doesn't really matter why. As long as it's compatible with the characters of both the hero and the villain, it's perhaps characterization, but it's not a plot point. Once he's run, he will find much better reasons to deal with the villain. It's not like the villain isn't capable of trying to murder the hero because he doesn't like the color of his eyes. (And prophesied dangers are being ruled out by authorial fiat -- hmm -- on reflect the villain wouldn't believe the oracle, so I can definitely rule them out, for reasons even.)
Nevertheless it's a sticky point on beginning the outline. (The rest of the story being much clearer.)
Of course, it could be that I just don't realize what significance it will have later, but then, those tend to profit from random throw-something-in inspiration.
For the moment, I am deeply pondering having the hero completely ignorant. Though that can lead to trouble, too.