First, good isn't knowable. We simply lack the perspective to understand how things fit together in the big picture. (And why's the big picture worth knowing?)
Second, why? Why anything? Good question; people invent reasons and then have wars over them. Because I want to; that's why.
And really, that's always why; there is always a choice, and people choose what they prefer. Preferences are formed based upon possible outcomes; views of the most desirable possible outcome are based upon beliefs; beliefs are based upon... what?
Uh huh. Well, either it comes from inside you or outside of you.
So. There's no way of knowing, and if we're honest, we just choose what we want anyway. That's true, but doesn't really inform how we live, or make a proposal for how we ought to live; good. The question isn't legitimate in the first place.
Related reading: I wrote a note to Larry in response to his post here, and quote part of it below. Take a look at his post and the discussion there.
It's ironic that when people feel very strongly about what's right (or wrong) and what should be (or should not) that they very often start a war. Deaths of persons, civilizations - and relationships. How many families are divided into factions over some thing or another? I don't mean out there, I mean here, in our own towns and cities.
If I say, perhaps, that right and wrong don't absolutely exist in knowable form, that they are words, descriptors, reflecting out of our experiences and perspective. If one believes that everything works to the greater good... then everything is good, even that which appears, even wars, to be evil.
Okay, I'm really bored. Splat.