Why would aliens come all this way just to invade Earth?

In movies, when aliens invade our planet, they usually have a ludicrous motivation. Like, they're after our water. Or our gold. (Hint: Asteroids have gold.) Space travel is incredibly difficult and expensive — so why would aliens actually bother to come and invade us? We asked some experts for reasons that actually… » 5/02/13 1:30pm 5/02/13 1:30pm