Question

Your friend could not be get selected tobrepresent the school at the group annual dance competition. he has become rude with all the other dance participants and dance teacher.

-what the reasons for this behaviour could be

Answer 1

He could be jealous, he may not know why he wasn't picked which may make him mad. He may also feel like everyone else is better then him since he didn't get picked

Answer 2

The main reasons would most likely be jealousy and anger.
Jealousy, because other dance participants were picked over your friend and allowed to participate. Anger, because your friend most likely believed that he had all the qualifications to be picked, and yet he was not.
Disappointment at not being picked, also fits in too.