Rabbi A.L. Scheinbaum - Peninim On the Torah
God turned to Abel and to his offering. But to Cain and to his offering God did not turn; this annoyed Cain exceedingly...And God said to Cain, "Why are you annoyed?"
Hashem is questioning Kayin about the source of his depression. Does not the Torah, however, in the preceding pasuk clearly state the reason for Kayin's annoyance: Hashem's rejection of his offereings. Is that not enough reason for one to be despondent? Horav Chaim Brisker, z.l., gives a practical explanation to the query. Hashem asked Kayin, "Why are you really upset? Is it because your korban, offering, was not accepted, or is there a deeper reason, the fact that your brother's korban was accepted?" Quite possibly Kayin's dejection was not due to his own rejection by the Almighty. Perhaps the real problem was the fact that his brother's korban was accepted.
Regrettably, this has become second-nature for many. They can tolerate their own lot, as long as their neighbor does not succeed. The thought that someone else succeeds is a bitter pill for many to swallow. We tend to become so obsessed with our friend's success that we neglect doing something about our own failure. How are we to correct our own mistakes when our prime concern is our neighbor's achievment?