Hiya Hogmeads: I thought that having Lupin's boggart turn into a full moon made great sense. He's a man who fears the full moon and it comes around every month. He is never free. For him the full moon means painful, crazed nights, waking up in a strange places, naked and bleeding wondering what or who he might have killed. It means being always longely, not for fear of being discovered, but because he might make friends, only to hunt them down when he turns into a werewolf. Being a werewolf is "...a cursed life. A half life" but in Lupin's case, not a life that HE chose.