Peter watched his supposed friends sadly. Lily and James, Sirius and Remus, all laughing and chatting and snogging each other (not all of them snogging one another, just their respective girlfriend or boyfriend), not even noticing they were leaving him out. Not seeing him sitting there on his own or that his smile wasn't actually a smile.
They probably didn't even care.
Unwanted.
Unnoticed.
An invisible fifth wheel.
Was it any wonder that every so often he casually slinked over, unnoticed, to a small group of Slytherins?
At least they appreciated him.
More than the Marauders did, at any rate.