It is unclear what motivates Nene has until the end of disc 2 where he reveals that he is tired of seeing children enjoying their youth while he is stuck inside an old body. To become young again Nene
captures Shu and the gang, and sucks the blue spheres out of them. Swallowing them himself he takes their blue dragons and uses them for himself. Once Nene has all of the blue dragons, he combines them to create an ultimate red dragon, which also allows him to go back to his younger self.