The Ugly Duckling – when is excluding someone mean, and when is it OK?

A group of ducklings decide one of their own just doesn’t fit in. He can’t keep up with their games, and — let’s be honest — kind of ruins the vibe. But is it fair to leave someone out just because they don’t have rizz? And is it even possible to create a close-knit group of friends without turning someone else into an outsider?