This is perhaps the most common and most basic human failing: We see someone else doing well, instead of working to do better ourselves, we instead want to take the other guy down.
Why does Cain feel so threatened by Abel? They're not even playing the same game - Abel was a shepherd, and Cain was a farmer. And G-d even tries to work with Cain: "If you do well, there is advancement; but if you do not do well, sin lurks at the door." (Gen. 4:7) (The Hebrew in the first part of the verse is tricky, and it is possible to translate it "Whether you sacrifice well, or whether you do not, sin lurks at the door.") I've used the more common translation; either way, it is clear that G-d is exhorting Cain to take responsibility for his own actions.