reasons

1. thinking with wrong head (someone came onto him & he was too weak to resist) 2. ego trip 3. not getting it at home 4. revenge (she cheated) 5. thinks he won't get caught 6. it's exciting 7. he doesn't love his significant other anymore 8. the challenge 9. significant other turns a blind eye to it 10. he no longer finds her attractive (age, weight, changes children made to her body, etc) 11. he thinks grass is greener on other side