He believed the shit people were shoveling him, barely trained, honestly thought his talent would beat anyone and no longer needed proper training camps or game plans.
When a fighter ditches his trainer, the one he's been with since 9 years old and got him to the top and decides his brothers and friends know better (they literally knew fuck all), he's going to get a rude awakening and Barrera was that awakening.
A Naseem with Brendan Ingles in his corner, training him and guiding him walks through Barrera but a Naseem that decides he knows everything and puts his dickhead brothers in charge deserves to fail.
He still managed to make a fight of it though, Naz could've been a legitimate legend, he was fucking insanely good.
If you're genuinely interested take a look at this documentary, it highlights exactly what an arrogant prick he became, I mean hiring a couple of Mexican kids to spar with in his villa for a couple of days...jesus.