Remus Lupin had always thought of himself as a rather optimistic person. You'd have to be, in order to survive transforming into a monster every each and month without completely losing your mind.
Others might describe him as calm, mild, or rather reserved, but considering everything that's gone on in his life and the fact that he hasn't yet committed suicide... It's no wonder he doesn't exactly wear his heart on his sleeve.
He certainly doesn't display his joy as James and Sirius used to, gallivanting about Hogwarts castle like madmen and blowing things up, but Remus likes to think that he exudes a quiet, subtle optimism. It's there. You just have to know what to look for.
He always knew he needed them far more than they needed him. His first full moon without them, after Lily, James, and Peter are dead and Sirius has been sent to Azkaban, is his worst yet.
He misses his friends. Remus was never quite sure of what he wanted in life, but he knows that he doesn't want this. Feeling very old and quite alone at the tender age of twenty-one, Remus suddenly doesn't feel optimistic anymore.