People cheat because they don't want to breakup or get divorced. Most cheaters aren't looking to replace one relationship with another.
The goal of any cheater is to hold onto all that is "good" in their primary relationship while addressing their other "needs" on the side.
Instead of being content with the 80/20 rule where one acknowledges their mate can't give them "everything" they want; a cheater attempts get the remainder on the side. There are 3 basic types of cheaters.
1. The Incessant Cheater - (Most likely has never been faithful long-term) Their motto is "Variety is the spice of life!" They become bored easily, love to flirt and use sexual innuendo with strangers. Oftentimes they're charming, good looking, smooth talking, and clamor to be the center of attention. They love "new adventures".
2. The Unbelievable Opportunity Cheater - Unlike the incessant cheater this guy is not on the look out for a "new thrill". Instead he may have a crush on someone and was content to "lust in his heart". However an opportunity arose where this individual made them self available or she was considered "out of league" but tonight she's flirting with him. This might occur on a business trip. Odds are high his mate/spouse will never find out. Of the 3 types of cheaters this is the one who may end up confessing weeks, months, or years later if they have a wonderful woman. The guilt eats at them overtime.
3. The Discontented Cheater - He has justified his cheating actions based upon his mate/spouse's behavior or treatment of him. Maybe she stopped having sex as much or no longer does certain things in the bedroom he likes, she nags and complains but never compliments him, she takes him for granted, she let her body go and stopped trying to look nice...etc
It doesn't take much for stranger to put a smile on the face and in the heart of an unhappy person. Some compliments or flirtation behavior is likely to make this guy want to be with her more.
However at the end of the day "Why" is never really important. In fact it's simply a "reflex" or "rhetorical" question a betrayed person asks.
In their mind there is NO reason he could give that would "justify" him cheating! Essentially people don't really care about "why". It's not as if someone is going to say; "Oh, I see why you did that now. Thanks for explaining it to me."
Each of us is entitled to have our own "deal breakers". If cheating is one of yours then you get out. It isn't you learn to look the other way I suppose.