Noctis seduces and starts a relationship with Cor because Cor reminds him of his dad and Noctis sees it as a way to get the paternal affection he can't from his real father.

Would prefer it without actual daddy kink though.
Bonus points for lots of emotional turmoil, like Cor worrying that he's overstepping his bounds and Noctis feeling guilty for manipulating Cor and not knowing if he actually has romantic feelings for him or not.