One of the most interesting explanations I've seen is that Western Europe was politically fragmented just enough so that big enough entities were competing with each other for dominance. So there was no central authority strong enough to pacify it, and the individual states were powerful enough to mobilize resources, creating a competitive power race. It was in trying to beat each other that they reached out and colonized the rest of the world.

Edit: I'm thinking now how during the apex of pax Americana, space exploration really subsided for example. When the US and the USSR were competing it was on. Now that US hegemony is declining, it's seems to be on again. Too strong of a political unification keeps the centrifugal forces in check.