A chance to mold Harry and do what he wanted Harry to do.
Yes! I just re-read that part, and it's that Lupin's instruction - "Think only of me" - struck me as basically priming Harry for a later event, linking Lupin and desire thoroughly. And Harry'd do it, of course, because of the newness of the bathroom event - you know, in that way that a new person is so vivid that the memory is almost physical - and because it'd surely arouse him like nothing else, so when it comes time for it, he's utterly ready. Awesome stuff :D.