Snape likes to define things, put a label on it, shove it a box, file it away- organize. He favors the high upstanding of his reputation and does not like bringing it down. Lupin is a free spirit. Doesn't have to organize- knows to enjoy life now because in a split second it can all be snatched away. Reputation never prevails over happiness.

"A guiding light out of this darkness is what he wants, but Sirius isn't there anymore to be that light. He may never be again."