וַיֹּ֥אמֶר יְהוָ֖ה אֶל־קָ֑יִן לָ֚מָּה חָ֣רָה לָ֔ךְ וְלָ֖מָּה נָפְל֥וּ פָנֶֽיךָ׃
Entonces SEÑOR dijo á Caín: ¿Por qué te has ensañado, y por qué se ha inmutado tu rostro?
Sforno on Genesis
Why are you angry. Why are you jealous that I accepted your brother’s offering? I did not do so arbitrarily nor was it unjust.
Radak on Genesis
'ויאמר ה, the reason that G’d addressed him was in order to discipline him and to bring him back to a lifestyle that would endear him to G’d. He wanted to teach him how to repent, something of the utmost importance for subsequent generations. He taught him that there is atonement for sinners if their repentance is 100% sincere.
Sforno on Genesis
Why are you depressed. It is pointless to brood over the past when the matter can be rectified