Peter wasn't quite convinced that Sirius was... well, serious. He could just barely wrap his mind around the concept of anyone wanting him, and since Sirius technically qualified as "anyone," it wasn't impossible. Still, it didn't quite make sense. "You're not supposed to want me. And you can't have me. Go want Moony or Prongs or Evans. There's a great big group of people you're permitted to want."
It probably should have been a concern that Peter's instinct was to worry about his friends' approval rather than Elizabeth, but he'd known them longer and, really, cared more about their approval.