In “Attack on Titan,” Eren Yeager’s cage was bigger than Father’s, but a cage nonetheless. Eren’s life behind walls makes him feel like a bird trapped in a cage (or cattle in a pen, depending on the translation). After his friend Armin showed him pictures of what the outside world has to offer, from the tallest mountains to the biggest oceans, his goal is to see those for himself. That’s initially why Eren wants to join the Survey Corps, Titan-killing soldiers who explore the outside. After his mother’s death, Eren’s resolve strengthens.
On his quest for revenge, Eren learns the truth about the world. The Titans are more human than they appear — Eren himself can become one, and the walls meant to safeguard mankind from Titans are made of rows of inert giants. The world outside the walls isn’t so empty, either.
In the season 3 finale, “The Other Side of the Wall,” the Survey Corps make it to the ocean after clearing the land of Titans. Eren finally gets a taste of his wish, but because of what he knows, he can’t enjoy it. Rather than freedom, more struggle awaits Eren and his friends on the other side of the sea. So, all he can do is look forward and ask, “If we did cross the sea and we killed our enemies … after that, would we finally be free?”
Eren decides he has to answer that question for himself.