Why Couldn't Eren Transform In Season 1 -

Eren’s mental state directly governs his power. In Season 1, he is a raw, traumatized teenager. Moments of intense despair, self-doubt, or rage can either trigger a transformation (e.g., when he sees Armin about to be eaten) or completely block it (e.g., when he doubts whether he is a monster or a hero). The most notable example is when he tries to transform to lift the boulder in Trost: he fails repeatedly until Mikasa’s words reframe his purpose from “transforming” to “saving everyone,” allowing his mind to clear.

In the dungeon, he failed because he was starving; the "engine" had no fuel. why couldn't eren transform in season 1

The shifter must sustain a physical wound that draws blood. This acts as the physical catalyst. Usually, Eren achieves this by biting his hand or thumb, though external injuries—like a blade wound or a gunshot—can also suffice. 2. A Clear, Fixed Goal Eren’s mental state directly governs his power

Eren Yeager’s struggle in Season 1 was the struggle of a human learning to be a monster. He thought he could just wear the skin. He didn't realize yet that to be a Titan, he had to sacrifice his humanity to the drive to survive. Once he accepted that—once he stopped trying to use the Titan and started becoming the Titan—the lightning finally struck. The most notable example is when he tries