Just why does the series have a pumped-up, Hot Blooded opening theme contrasting with a soft rock ending? The final episode drives the point home - Yuusuke, finally unburdened by the duty of being Kuuga, shows his kindness to a group of children with the ending theme playing over. The two musical themes of the show present the character of Yuusuke Godai - a kind man forced by circumstances to become a fearsome warrior who would save the world.
The final Good Old Fisticuffs battle, suddenly you realized that N-Daguva-Zeba wasn't fighting to win, he was hoping that Yuusuke loses what's left of his humanity in that fight. Kuuga was also a Grongi himself (an immature one) and Yuusuke was simply the human host for the trasnformation. The final fight was to truly bring out Kuuga as a Grongi so he can usher in an eternity of violence.