Genesis 4:5

"But on Cain and his offering he did not look with favor.  So Cain was very angry, and his face was downcast."  Genesis 4:5

So here it tells us that God did not look with favor on Cain's offering.  We don't know why.  Was it just not good enough?  Was it because it was not animals? 

I'm inclined to believe it had more to do with where Cain's heart was.  Verse 4 says that Abel brought God his best, but it doesn't describe Cain's offering.  Perhaps Cain's offering were an after thought - "Oh yeah, I should give back to God, here's some harvest I have left over that I can spare..." or maybe Cain was just being selfish and wanted to keep the best to himself.  Or maybe it was simply Cain going through the motions.

Whatever the reason, Abel's offering was better than Cain's and God was pleased with Abel.  It doesn't say God was angry at Cain - just that he did not look with favor on Cain.  Because Abel's offering was better, Cain was filled with anger. 

The Qbible translates "very" as "wholy" or "speedily" and "anger" can be translated as "jealousy."  So this was a jealous anger that raged through his whole being.  It's hard not to feel that anger sometimes, even when we know that the other person is deserving of the respect and honor they are receiving and we are not.  Emotions don't follow logic.  The real question is how will we react to those emotions?  Will we allow the anger to control us?  Or will we control the anger?