וַיֹּ֥אמֶר יְהוָ֖ה אֶל־קָ֑יִן לָ֚מָּה חָ֣רָה לָ֔ךְ וְלָ֖מָּה נָפְל֥וּ פָנֶֽיךָ׃
And the LORD said unto Cain: ‘Why art thou wroth? and why is thy countenance fallen?
Read kabbalistic (mystical) interpretation of Genesis 4:6: and classical mystical sources.