Dog trying out its new legs

So much waggle!

A dog in pain has its tail between its legs and its ears down, this one is happy as can be

Exactly. He's just having to learn how to walk with his new legs. They probably feel really weird to him right now but he'll adjust.

This is one happy dog. If he was in pain, his ears would be back with his head bowed, his tail between his legs, and his hackles would be raised. He's instead very energetic, with his ears up and his tail wagging while down. He's very excited, but somewhat nervous. He's likely nervous because his new legs feel strange, and he has to learn to adjust his gait. But otherwise, he's as @anthracite said, as happy as can be.