Cain became angry because God regarded Abel and his offering but did not regard Cain and his offering Genesis 4:4–5. God warned Cain that sin was “crouching at the door” and that he must rule over it Genesis 4:6–7.
Genesis 4:1–7 introduces the first two brothers, Cain and Abel, and the first recorded act of worship through offerings. Both bring gifts to Yahweh, but Yahweh regards Abel and his offering, not Cain and his offering. Cain’s anger shows in his face. This scene follows the expulsion from Eden and the beginning of life outside the garden, where work, pain, and relational strain now mark ordinary existence (Genesis 3:16; Genesis 3:17). Genesis 4 moves from the first family’s growth to the first major breakdown between siblings.
Genesis 4:1–7 presents the first set of siblings and the first narrated offerings to Yahweh. Cain and Abel each bring a gift tied to their work (ground produce; flock animals). The text explicitly says Yahweh “regards” Abel and his offering but does not regard Cain and his offering. Cain’s anger shows outwardly in his fallen expression.
A careful answer keeps the immediate passage, the book's argument, and the whole-Bible storyline together, so the question is answered in context instead of as a detached idea.
Start with Genesis 4:4-5, then read the surrounding section so the answer stays connected to the passage's flow.
4Abel also brought some of the firstborn of his flock and of the fat of it. Yahweh respected Abel and his offering,
5but he didn`t respect Cain and his offering. Cain was very angry, and the expression on his face fell.