Prompto was neglected basically all his life. He had to gend for himself, pay bills and do whatever he could and his parents were always away except for like a few days.

++When the parents are home, they just find Prom to be an annoyance, constantly telling him to leave him alone

+++Always mad at him for something

++++The bros unfortunately show up at his house one day and witness Prom's dad slapping him across the face

Basically Prom has really shitty parents and the bros are appalled their little ball of sunshine would put up with this