Stephen Hawking - Aliens

Even if they do, immediately attack us, they would have done so, anyway, when they came here on their own. If they will destroy us, why delay it?

Because the longer we survive, the more we will evolve, and the more likely we can defend ourselves and have control over the situation. Ultimately we, or our planet, is just another organism fighting for survival. And from what I've seen in nature, weaker organisms tend to try to hide themselves. I think it would be safe to assume that if there is intelligent life beyond our little box, it's likely to be far more advanced than we are. In the grand scheme of things, we're in our infancy. Hell, we're all alone on our little ball and we are still barely managing to keep ourselves alive. I think we've got a lot of work to do before we should start advertising our existence. I'm not saying we should actually hide and not respond to any attempts that other life might make to contact us, though.