Why did God reject Cain's offering while accepting Abel's?

We do not have much to go on in the text, but something sticks out: the criterion for offering God a sacrifice, mentioned many times in the Pentateuch, is that it has to be the first of your produce, not the last, not whatever is left over after you have eaten your fill. Abel gave the firstlings of his flock; he went out and picked the best he could find and sacrificed that to God, and then kept the rest. But it says Cain gave an offering of the fruit of the ground; he had it backwards, eating his fill and then giving God the leftovers. God does not want leftovers, and when God is insulted, watch out. That is why Cain was rejected, and of course his reaction was to be upset at the Lord, how could You do this, which is the normal reaction of someone trying to cover his tracks, even trying to gaslight God as though God were the guilty party. So the rest of the Pentateuch confirms it: when you give an offering to God, make sure it is the first of your flock and without blemish, the best you have. That is real sacrifice, and that is what Cain failed to give.