Why does Cain get angry in the first place?

Because God likes Abel's offering of meat (and possible milk), but turns his nose down at Cain's offering of fruit and grain. The first time I heard that-and I would have been quite young, it struck me as discrimination, pure and simlpe. I mean Cain was just trying to make a living, and God, Lord and Master of the universe, behaves in a completely arbitrary and/or petty manner. Now, what is this supposed to prove? Wouldn't God have known that Cain would slay Abel? This, I think implacates God.