What is more surprising is that Eren visibly holds himself back - opening his mouth and then shutting it just as quickly. Immediately, inexplicable, Bertholdt feels guilty. Here Eren is obviously trying, and Bertholdt is annoyed with him.

(But what had he been about to say?)

He pushes it down, and aside. Feeling guilty for Eren is a black hole that one will never crawl out of, if they're not wary. The on purpose barb, about the wall, is almost a comfort. Something intentional, personal, but that he expected.

"Thanks," he says, just a hint of sarcasm in his tone despite himself - but he doesn't try to stop Eren or follow him. He doesn't offer a rejoinder, and if they have nothing to be sad to say to each other, he lets him go.