There is a fact that is poorly explained by most theories, and that is the fact that most predators avoid humans and humans are most definitely not preferred prey for most of them; this is why "maneaters" among lions and tigers, for example, are rare. This despite the fact that humans are weak (much weaker than chimps, for example), slow over any distance less than a half-marathon, and have a very poor sense of smell.
I think the reason why lions (for example) don't normally hunt humans is fundamentally that humans have the intellectual capacity to bear a grudge - and we often do. Which means that a lion with a taste for human flesh is probably not going to breed, at least after it has indulged the urge. Other prey animals, even those which have decent natural weapons (such as various antelopes and buffalo) don't hunt down a lion that kills one of them. We do.
On a somewhat more SF note, ETs might well be put off communicating with us for various reasons. Just one of those is that a large proportion of the population spends many hours running combat simulations - for fun (eek!) - and a fairly large part of the economy is connected to programming them and also making the hardware to run the sims better.
