This would make sense if Snape and Lupin had been friends at school or simply hadn't known each other at all and are only now establishing a relationship. However, it would take a really dense person not to realise that Snape hates him and Lupin certainly isn't dense. In view of what we know from OotP I can only see it as him rubbing his presence on Snape's face. Unless he thinks that Snape has lovely memories from High School and all that great fun the Marauders used to have with him.