1. He felt nervous. Dancing in front of others can be intimidating for some people, and he may have second thoughts once he was actually in the moment.
2. He thought better of it. He might have realized that he didn't really want to dance, or that he wasn't in the mood.
3. He was trying to impress you. Some people might ask someone to dance as a way to flirt or show off, but then they lose interest once they realize that it's not going to lead anywhere.
4. He was distracted. He might have been distracted by something else that caught his attention, like a friend he saw across the room.
5. He was just being polite. He might have asked you to dance out of politeness, even if he didn't really want to, and then changed his mind once he realized that you were interested in dancing.
It's important not to take his rejection personally. There are many reasons why he might have changed his mind, and it's not a reflection on you. If you're feeling disappointed or hurt, it's okay to let your friends or family know how you're feeling. They can offer support and encouragement.