I think the story was good but it would be much deeper if Bowser actually is a brother of Mario and Luigi.
M & L's father cheated on his wife with the Koopa maid and she gave birth to Bowser. The father sent the maid away and sent Bowser out in the forest to die, but he was found by Kamek who raised him.
In his early years Bowser struggles to become something great, and after a lot of effort he becomes king. He also fell in love with Princess Peach. Peach and M & L are close friends but Bowser gets no attention at all.
He locks himself up in his castle. Then Kamek reveals his past, that he is the brother of M & L. Bowser begins to hate them. They have lived an easy, harmonic life. Bowser had to fight to even get something to eat. He has lived a life in misery and pain in the outskirts of the world. M & L lived in Mushroom Kingdom were every day is sunny and everyone has food on their table.
His anger makes him declare war on Mushroom Kingdom. He is going to take away all the great things M & L had that Bowser only could've dreamed about. He will steal the girl they love, princess Peach and marry her. Then his victory will be complete.
