Such a believable, bittersweet tale. It would make sense that there was this reason, as well as the other one, for Lupin to leave Hogwarts. They are all so well-characterized; Lupin with his reluctant admiration for Snape's courage, and the way Snape just can't stop snarling.
And Minerva, who wouldn't regret the wartime romance, but who is now firmly with Snape, however exasperating he can be.
And poor Remus, so very good at not saying things and not showing his feelings. The poor man.