In a kid's book of Bible stories, it's resentment over having to make sacrificies. In the movie "The Bible," it's stinginess in sacrifice. In each case, it's the reason that YHWH rejects Cain's sacrifice.
But here's what Genesis says about YHWH rejecting Cain's sacrifice:
Abel was a shepherd, but Cain was a farmer. In time it came to pass that Cain offered the fruit of the ground to YHWH as sacrifice. And Abel sacrificed the firstlings of his flock, even the fat. And YHWH respected Abel and his sacrifice. But He did not respect Cain or his sacrifice. (Gen. 4:2-5)
No reason is given for YHWH favoring Abel's sacrifice over Cain's, other than that one was flesh and the other grain. Maybe it made sense to the nomadic, sheep-herding Hebrews that their god preferred the sacrifice of sheep to the sacrifice of grain. After all, their urban, decadent enemies were mostly farmers.
To the modern reader, however, the idea of blood sacrifice is repugnant. It makes no sense to us that YHWH would rather that we kill sheep in his honor than burn grain. So modern interpreters invent sins for Cain, such as resentment or stinginess.