Levi did not actually agree with Erwin in this instance. If Levi's commander went and disappeared from this place, it wouldn't bring him peace knowing he was dead, just bones on the bed that Levi had laid him to rest in. Not knowing wasn't any worse than that.
But it wasn't about later. Levi had decided that already. It was about now and having what they hadn't allowed themselves before -- if the amount of time they had for it was questionable or limited, it was still more than it had been.
So Erwin could be comforted how he liked, and Levi would do the same. "I'll handle it how I best need to," he said, which was true. He'd carried on under great loss and stress before, and he'd be able to do it again. Even if he didn't want to.
So he took the hand offered to him, calloused fingers brushing over the much larger palm of Erwin's hand before he slotted their fingers together.