As soon as Harry leaves the classroom, Remus sinks heavily into a chair and succumbs to the aftermath of facing Harry's boggart. Part of him knows that he probably shouldn't have agreed to help Harry with his dementor problem, not when he can hardly bear the effects of the foul beings himself. While he can conjure a patronus, it was no lie when he told Harry that he was hardly an expert in fighting off dementors. Still, he's much better at masking the ill effects than Harry is. Decades of hopelessness and despair have taught him how to fake it, after all. But despite any façade he may put up to prevent others from knowing how weak he truly is, he cannot fool himself.
Remus sighs and breaks off a piece of chocolate. Bringing the sweet to his mouth, he tries to rid himself of the cold and despairing memories that threaten to flood his mind once more: the terrible pain of his bite, and the even worse agony of his first transformation; the betrayal, not just once, but twice by Sirius; the deaths of Lily and James and Peter… Even with the boggart gone, they replay over and over in rapid succession as a terrible ache builds in his chest, forcing air from his lungs.
"No," Remus says aloud, forcing the forgotten piece of chocolate into his mouth and willing the memories to return to the recesses of his mind where he keeps them locked up. Remus exhales and swallows. He should definitely not be spending any more time around dementors—even fake ones—than absolutely necessary.
Had it been any other student asking for help, Remus is sure he would have declined, would have claimed a tight schedule or an ill relative he had to attend to (Merlin knows he's used that excuse several times over the years). But it had been Harry. Lily and James' Harry. The young man he held in his arm as a baby but didn't get to see grow up.
And it kills Remus to see him every day, to teach him in class and watch him with his friends, and not be able to be the uncle he was meant to be. It's his own cowardice—cowardice and shame—that keep him from seeking Harry out and telling him everything he can remember about his days at Hogwarts, about his adventures with James and Peter (and, yes, Sirius), about his mother's kind smile and intelligent eyes, about the fierce unwavering loyalty of his father. But he's afraid that Harry doesn't need him in his life, that his presence—should his affliction ever get out—would be a burden. He doesn't think he could handle that rejection, not after everything. Not from James' son. And he's ashamed, fraught with guilt over the fact that he didn't realize that Sirius was the spy, that he allowed his best friends to be murdered because he hadn't read the signs that Sirius Black was working for Voldemort. That he had let Dumbledore take Harry away from his true family and hide him away with those muggles.
No, there is no way that he could deny Harry this request, he realizes, not with what the dementors force the boy to hear. Remus takes another bite of chocolate and feels the cold slowly begin to fade. Against his better judgment, Remus can face Harry's boggart with him. For Harry—for James and Lily—he can stomach the weekly reemergence of his own horrific past. For the Potters, Remus could probably do anything.