beta
There's a question of whether Brett's "doing too much" when his relationship with Hayden can sometimes seem like a casual fwb agreement. But if you go back to Ch. 5-6 when they're in the library, it was explained that Brett had to go through intensive investigation (stalking) to find someone like Hayden. His obsession stems from his long standing dissatisfaction of his life, the people around him, and the role he's continuously expected to play as the alpha son of a rich and prominent family. Therefore when he finally found Hayden he became the sole thing that allowed him to be and experience the very thing that gave him happiness and the freedom to enjoy sex the way he wants it.
I'm not sure if either Brett doesnt believe in love, or simply sees it as something fleeting so he doesnt see the point in getting invested in something temporary. But its possible that part of his feelings for Hayden may already be love, he just doesn't know it yet.