Why do people cheat when they're in a relationship?

The cheater was never committed to being in a relationship anyway.

Generally because they feel neglected and unfulfilled in their relationship. Sometimes it happens cause they got drunk or they're just mean-spirited people.

Variety
Ego boost
Boredom
Selfishness

They feel they've been dealt the bottom of the deck

Because they can. No one, however, should accept getting cheated on.

Cuz that ass is better than that one.