Wroth

4:5 But unto Cain and to his offering he had not respect. And Cain was very wroth, and his countenance fell. 

I'd be wroth, too; wouldn't you?  I think that wroth means "pissed off".  And "his countenance fell" means "he looked sad."

Ok, Cain grew fruit and Abel grew sheep.  And Cain offered up to God some fruit and Abel some sheep.  Why they made offerings to someone who so badly mistreated their parents is one question.  Regardless, they clearly offered what they had to offer.

God, that day, was apparently not in the mood for fruit.  And he was rude about it.  No explanation is given; the only option we have is simply assuming that God was being a jerk or a troublemaker.  And he mistreated Cain, which seriously bummed Cain out.