So Isayama came out with a blog post that talks about a childhood friend he has. What I took away from that is him explaining his inspiration for Armin, and how he’s, in a way, a pure soul because he never lost sight of things that were most important to him as a child. There’s a sense of admiration for someone who is so headfast in his desire to obtain the things he wanted as a kid. And, to Isayama who has outgrown those desires, he kind of feels bad about himself.
I could see this as a parallel between Eren and Armin’s relationship, actually. It might be a clue into where things are with these two, especially after chapter 90.
A lot of people feel Eren’s lackluster reaction to the ocean is worrisome, and rightfully so. But this kind of goes back to what Eren was saying when he begged Levi to save Armin. He saw Armin as someone better than him, someone pure and unwavering in his desire to be free and see the ocean. He said he was terrible because he forgot those dreams and focused on something else.
I could say it’s the same with his reaction to the ocean. It’s not that he doesn’t care, and I don’t necessarily think he’s outgrown it either. His head is an absolute mess, and it’s very possible he was having his father’s memory flashbacks even at the ocean. He’s thinking maybe 15 steps ahead of where they are now, and if he took a moment to look at Armin and Mikasa to see their joy…he’d feel guilty; as if a part of him and his childhood has gone away because he can’t focus on just their dreams anymore. He’s so focused on other things.
Isayama said he used to be into things like Yu-Gi-Oh (tsh, what a nerd), but he’s not anymore. And yet he tries to still be that person that loves that stuff with his friend. That might be how Eren was before they left for Shiganshina, and I bet that’s what startled Armin so much when he saw Eren’s half hearted response about the ocean. But the moment Armin began to doubt the outside world, Eren was there to pick him back up. It could’ve been for selfish reasons (trying to keep that purity alive so he feels he’s still pure in some way too), but it could also be he just doesn’t want Armin to lose his goals. Without goals and dreams, you become nothing.