The text withholds the reason.
Genesis 4 says that the Lord had respect for Abel and his offering and not for Cain and his. It does not say why. Readers have supplied reasons for centuries, and the passage supports none of them outright. What it gives is the outcome and Cain's face falling in response to it.
That silence is doing work in the story. A verdict you can explain is something you can argue with, meet, or fix. A verdict with no stated basis leaves nothing to act on, and the feeling it produces has nowhere to go. The story begins with a grievance that cannot be resolved by understanding it better.
Anger arrives before the decision.
The text separates two things carefully. Cain is very angry and his countenance falls, and that is where the passage pauses. The killing is several verses away, in another place, after a conversation. Nothing in the account treats the first as identical to the second.
The warning lands in that gap. It does not tell Cain to stop feeling what he feels, and it does not tell him the feeling is harmless. It says the impulse is at the door and that he may rule over it. The story's interest is in the interval, which is the only place anything can still be decided.
Resentment arrives with an explanation attached.
People rarely experience themselves as simply envious. The feeling tends to present itself already reasoned: the standard was unfair, the other person was favored, the whole thing was rigged from the start. Cain's grievance has an unusual advantage here, because the text really does decline to justify the outcome.
This is where a psychological reading has to be careful rather than clever. Having a real grievance and being entitled to act on it are different questions, and the story keeps them apart. Cain is not corrected for being angry. He is warned about what he is preparing to do with it.
The question that avoids the answer.
Asked a direct question about where his brother is, Cain gives a denial and then a question of his own about whether keeping his brother was ever his job. The reply does two things at once: it refuses the fact, and it disputes the standard being applied. Neither requires him to say what happened.
The passage does not end with him erased. He is sent out as a wanderer, he says the punishment is more than he can bear, and he is given a mark so that no one who finds him will kill him. The consequence and the protection arrive together, which is harder to summarize than either would be alone.

