The Question

וַיֹּ֥אמֶר י״י֖ אֶל־קָ֑יִן לָ֚מָּה חָ֣רָה לָ֔ךְ וְלָ֖מָּה נָפְל֥וּ פָנֶֽיךָ

What is the question, Kayin was upset because his korban was not accepted?  Furthermore, what is the point of asking this question? 

The Baley Mussar derive from here two lessons.  Lesson one is that even though one is supposed to feel bad over the sins they have committed, חרטה על העבר, one can not get stuck in a state of depression over one's faults.  One's misgivings over past failures is only positive if it leads one to improve in the future.  It one merely gets gives up because of their past crimes then it is not good to be caught up in the past.  Kayan was so distraught over what happene that he could not move forward.  That is what the Sforno means ולמה נפלו פניך – כי כשיש לקלקול איזו תקנה אין ראוי להצטער על מה שעבר, אבל ראוי להשתדל להשיג תקון לעתיד.  One's goal must be to fix the problem going forward (see Chidashay Halev.) 

The other lesson is that G-d went to ask Kayin why are you down?  It is fitting for one to inquire about why one's fellow is down and ask if there is any possible remedy.  The Torah is teaching us that Hashme did this and that is the method in which a person should follow.  That is why Yosef asked his fellow inmates why they appeared depressed for he learnt this lesson from this parsha.  Rav Eliyahu Baruch wants to suggest that this goes so far as to that may be why Hevel was fitting to be killed by Kayin.  Where did he go wrong that he should deserve to be killed?  Because he did not come to comfort Kayin in his time of need.