Looks like you stumped the band, Ray; I had to do some major research into the whys and wherefores - even called a dear friend of mine, Prof. Puaolini, to discuss some of the finer points before getting back to you.
Believe it or not, getting tossed into space is how we got here in the first place! I know... I know. You're skeptical, aren't you? So was I! But Puaolini confirmed it. Our ancestors - farther back than you can imagine - were flung out into space and traveled here on the backs of star stuff.
Darwin had it right, y'know? Genetic memory being what it is, our ancestors tried for thousands of years to develop just the right 'attraction' to this planet to be able to hang on, as it were. Those who didn't develop the right 'attraction' got flung off Earth and ended up - oh, who knows where? Those who did develop the right 'attraction' stuck to the surface of Earth, were able to procreate, and here we are.
Occasionally there's a throwback to the old days and some of us are born with out that attractive gene we need to be able to hang tight - and over the years we build up speed and one day just fly off. Suddenly. Without warning. If you've ever been talking to someone who just up and disappeared you know what I mean.
And that's it! It's all genetics.
Why is water wet?