Let’s appreciate this beautiful scene. Mikasa is sitting next to Eren, in a very typical way which a person waiting by a loved one’s sickbed does, Jean is standing the furthest from Eren, but is till in the room with a worried body language, and Armin has taken his shoes off and curled into a ball on top of the bed.
Armin is in this panel worried about the titans living in the walls, and his mind is filled with thoughts of them. He is worried about what their existence means (“we’ve always been protected by titans from titans?”) and so he sits closely to Eren to find some stability. Even though he has berated himself for depending on Eren in the past, it is logical to assume that he sees Eren’s life as a source of stability, as Eren has always been his sort of saviour, and the person he is closest to.
Mostly I just love this scene because of how intimate it is. Armin so openly sitting in Eren’s bed is touching, and proves how comfortable he is with being close to Eren, even (semi-)publicly. This is anecdotal evidence, but there are not many male best friends who would sit like Armin does in their best friend’s beds, so vulnerably.
Armin is a character who has cried openly at many occasions, and is a boy who is in touch with his emotions in ways male characters are commonly not depicted as being, unless it’s being played for laughs. This scene is an example which proves how comfortable he is with showing worry and dependence on his best friend. It would have been very easy for him to sit like Mikasa (who hides her true worry until everyone else has left the room, when she moves her chair much closer to Eren), or stand up like Jean (who is trying to look tougher than he is feeling, as seen from his body language). But instead, he took off his shoes and got onto the bed, showing how much he worries and how close he feels to Eren openly, probably since it is natural for him to do so.
And I’ve got nothing more to add, really. I simply love this panel.