Well intentioned or not, his good mood doesn't seem to survive that little tip. He frowns. Sets his jaw, and his posture edges toward defensive, or combative, or both, because the two often end up becoming one and the same before long. Eren's not a thug, he's not even that bad a guy once you get to know him, but he's gone through much of his life with a chip on his shoulder. Spent years getting into fights with people bigger than him even before he had cause to carry a monster-sized grudge. (And whether or not he'd won those fights is beside the point.) He's a firm believer that you need to stand up for what you believe in. And there's nothing wrong with doing it.

"We're all fighting for a reason," he maintains, stubborn. This is a big day for him. Despite what he's been through he's an idealist at heart, and he doesn't do things in halves. But it's actually not too often that his energy gets to be expounded in such a positive way—if not one that might be universally accepted as such. They're on their way to the Academy, taking a big step toward his dreams, toward fighting for humanity—is he supposed to be unhappy about that? "For humanity. No one's being forced to come along."