It's how the Doctor can tell the companions from the cattle. He always spots the ones that look up.
So much of early man's destruction came from below; snakes, stalking predators, earthquakes, floods. You remember, don't you? The Doctor always finds the ones who can see where the potential destruction is going to come from, from above. From aliens bent on stealing our pretty green planet out from under us. Or, even just the normal evil humans, plotting alone in their ivory towers. He looks for the ones who look up with knowing in their eyes.
If it always just so happens that they're young and pretty, all the better, eh?