Boys/teens cheat because they're not truly ready/mature enough to settle down.
The goal of most cheaters is to hold onto all that is "good" in their primary relationship while addressing their other "needs" on the side.
Most cheaters have no interest in replacing their mate.
There are 3 basic types of cheaters
1. The Incessant Cheater
This person gets easily bored! They're always on the look out for someone NEW. They're usually very flirtatious and personable. Their motto is: "Variety is the spice of life.
(They will never be faithful long-term!)
2. The Unbelievable Opportunity Cheater".
Unlike the (incessant) cheater this one is not proactively looking to cheat. They may however have a secret crush on someone or have lusted in their heart after someone. However it's not until that person comes on to them that they realize they can turn a fantasy into reality. Oftentimes it happens on a business trip or under some other circumstances where their spouse/mate is unlikely to find out.
(They caved into temptation!)
It's not uncommon for this type of cheater to confess weeks, months, or years later to get the weight of the guilt off of their shoulders. They'll plead for forgiveness.
3. The Discontented Cheater
He blames YOU for why he cheated. You either stopped doing something or you started doing something, neglected them sexually, always complained and never complemented, took them for granted, weren't emotionally available, changed your physical appearance dramatically, or you were unsupportive of their dreams.
All it took was someone to "compliment" them to put a smile on their face and cause them to "feel special" again. A secret friendship developed and "one thing led to another". Naturally the majority of cheaters that get caught will swear they belong to this group.
It's the only one where the betrayed person might buy into the belief they caused their spouse to cheat and thereby be willing to forgive them. If cheating is a "deal breaker" for you then move on.
Both cheating and monogamy are (choices) individuals make!
Asking "why" is a rhetorical question. It's not as if there is anything he could say that would cause you to agree with the decision he made! Some people ask (why) because deep down they're hoping he will say something that will make them want to stay in the relationship. Others just want to see him squirm or dance through some excuses when they already know they're leaving him.