I think Heinlein summed it up most concisely, but the short form was from Krane:
A man said to the universe:
"Sir, I exist!"
 "However," replied the universe,
 "The fact has not created in me, A sense of obligation."
-In short, nature is Cthulhu. Just be glad it isn't [i]trying[/i] to kill us; [i]it still does[/i], in very horriffic ways.

[url=http://www.projectrho.com/public_html/rocket/spacewarintro.php#id--Ain'ta_Gonna_Study_War_No_More]Heinlein's thesis regarding the clash of intellegent animals to create a post-scarcity population-inversion:[/url]
[quote]But it was interesting. I caught one of those master's thesis assignments he chucked around so casually; I had suggested that the Crusades were different from most wars. I got sawed off and handed this: Required: to prove that war and moral perfection derive from the same genetic inheritance.

Briefly, thus: All wars arise from population pressure. (Yes, even the Crusades, though you have to dig into trade routes and birth rate and several other things to prove it.) Morals - all correct moral rules derive from the instinct to survive; moral behavior is survival behavior above the individual level - as in a father who dies to save his children. But since population pressure results from the process of surviving through others, then war, because it results from population pressure, derives from the same inherited instinct which produces all moral rules suitable for human beings.
. . .
Nevertheless, let's assume that the human race manages to balance birth and death, just right to fit its own planets, and thereby becomes peaceful. What happens?

Soon (about next Wednesday) the Bugs move in, kill off this breed which "ain'ta gonna study war no more" and the universe forgets us. Which still may happen. Either we spread and wipe out the Bugs, or they spread and wipe us out - because both races are tough and smart and want the same real estate.

Do you know how fast population pressure could cause us to fill the entire universe shoulder to shoulder? The answer will astound you, just the flicker of an eye in terms of the age of our race.

Try it - it's a compound-interest expansion.

But does Man have any "right" to spread through the universe?

Man is what he is, a wild animal with the will to survive, and (so far) the ability, against all competition. Unless one accepts that, anything one says about morals, war, politics - you name it - is nonsense. Correct morals arise from knowing what Man is - not what do gooders and well-meaning old Aunt Nellies would like him to be.

The universe will let us know - later - whether or not Man has any "right" to expand through it.[/quote]

I'm not quite to the point of murding all horses and ponies, just to get rid of horse-flies, but I'm pretty close to it.