So far in the series, Eren has gone to great lengths to keep his foster sister Mikasa at arm's length, despite signs that he actually cares for her. However, a recent flashback in the Season 4, Part 2 finale makes it abundantly clear why Eren has gone out of his way to make it seem like he has nothing but contempt for the girl.

In the flashback, Eren’s half-brother Zeke says that it’s Mikasa’s love for him that triggers her powers. This thought prompts Eren to emphasize that he only has four years left to live, and that he wants everyone who cares about him to be at peace with his death, once he’s gone.

As a matter of fact, back in the present, he doubles down so fiercely on his hatred for her that Armin physically assaults him. However, the flashback confirms what the audience has long suspected, that Eren is in deep denial about his true feelings for Mikasa.