"There is a finite difference between being stubborn and hiding in a plummeting spiral of self-pity and despair," Severus complains, "the latter of which Lupin is a bloody professional at. Passive-aggressive responsibility-dodging arsehole."

Tell us how you really feel, Snape.

"Looking for people out in the ship, that fool. I don't know why I even bother to check up on him."

Because he made a promise, basically, an no matter how much of a prick Severus is, he keeps his word. Even if he seems really, really pissed off about it right now.