And beyond the chemical issues, there's a very basic reason for this: There's no way an established relationship will ever be like a new relationship. New relationships are exciting, passionate, hopeful and mysterious for the reasons I've listed previously. After a little while, a shiny new relationship will be old and established. They move past that phase to a place of companionship, trust and security. Perhaps; it's part of the transition from new relationship to established one.
But with dedication, passion doesn't have to wane — it can stay and grow with the relationship.
Moreover, his dissatisfaction isn't necessarily a deal-breaker for an established relationship. Maybe he did discuss those issues with his wife, but perhaps his version of "discussed" meant he "screamed at her over and over and over and over again" while fighting off endless-but-completely-accurate assumptions that he was disconnecting from her and possibly seeing someone behind her back.
The reality is very simple: His discontent wasn't necessarily impossible to overcome; he just needed to talk to his wife.
Then he found some model/starlet/socialite who had more firmness in her tits than morals in her fiber. After a few shots of tequila, a man with any amount of conscience will tell you that he feels like he left his integrity and honor somewhere between watching the first condom twist its way down the hotel toilet and standing face-to-face with his wife while screaming, all with his kids within earshot. She's kinda flirty with him, sitting in front of his desk in her skin-tight blouse (the kind of sexy shirt his wife refuses to wear), and damn, she looks like she'd be fantastic in bed.
Celebrity journalists — oxymorons in their own right — highlight the latest indiscretion of some mega-rich, mega-famous husband who had what appeared to be a great marriage and a couple of kids. But don't be fooled — men like this have massive regret.
If he had any, he would've had the common decency to leave his current partner before ending up in a bed (or the backseat) with someone else. Nowadays celebrities ain't got nothin' on the extramarital habits of run-of-the-mill suburban dads.
The dopamine levels in his brain bordered on psychosis levels.
All he could think of was her — her touch, her taste, her smell, her laugh, the sound of her voice, her witty retorts — and everything in his life revolved around her. He was thinking nothing, and therein lays the issue.
But that's impossible when he's in bed with someone else and that new-relationship-feeling is bending his brain.
New relationships are this magical thing that ensure procreation so the human race lives on.