Bowser Jr was getting tired of being all kooped up in the castle all day, no pun intended. Actually getting bored of all the games he played, winning at each of them ofcourse. He admires his dad so much at what he does but sometimes hes just tired of it, being all repetitive. Just for one time he wants to see the Mushroom Kingdom without being destroyed so he goes but treated like dirt cuz of his reputation all he wants is some dang cake is that too much to ask? Btw just saying Not a ship cuz ew wth Just them gaining a friendship without bowser knowing