Barebacking is no different than any other addiction whether it be alcohol, drugs, or gambling.
First and foremost is the social programming that teaches us it's "wrong" or "inappropriate" that plants the first seed of desire.
Second, regardless of the circumstances involved, when we do cross that line for the first time it feels so fucking good that it stimulates previously untapped pleasure centers in our brain. Humans are hardwired to experience this type of pleasure regardless of the consequences - it just feels so good to be "bad".
We respond by trying to resist and not repeating the experience but it's too late. We crave that sensation of pleasure derived from forbidden behavior whether it be getting drunk, high, hitting the jackpot, or taking another man's load up our ass.
We're well on our way to addiction now and our desire for pleasure will start to rule our actions. The addictive behavior will become our norm and we will downplay any of the risks we previously associated with the behavior just to experience that pleasure once again.
Not a scientist or medical professional, just speaking from my own experience. Resisted drugs for many years but once I mellowed out on a little weed it was a feeling I wanted to experience on a regular basis and continue to enjoy despite the fact that it illegal where I live and could cost me my career if I ever get selected for random drug testing. Same with barebacking, played safe for many years but once I experienced the pleasure of a man cumming inside me (was high at the time and it seemed like a good idea) I was hooked and and powerless over my addiction. I enjoy a wide variety of gay sex but nothing gives me the ultimate pleasure I experience from a man draining his balls inside my ass and bottoming for raw cock is my "go to" activity when I need to get off. I no longer give a shit about the possible consequences, I just want to feel that euphoric rush a hot load of cum in my ass gives me so I can experience a mind blowing orgasm.