1. They aren't truly in love
Being in love can be confusing. Sometimes the feeling you once had about the other person may no longer be apparent. They are probably in love with someone else but don't want to hurt you.
2. They want to experience another person
I've known married people who thought about cheating on their significant other because of their sexual experience. Marrying at a young age can be the reason many people cheat on their spouses. It's not because they don't love them, it's because they haven't experienced another man or woman. Life is short and imagine not being able to experience all of it's sensations because you made a commitment.
3. They aren't loyal
While a significant other may be madly in love with someone, sexual desires can be strong. Other men and women can be very malicious with their intentions, therefore making a lonely spouse an easy hit-and-quit.
4. They just wanted a quick nut
Though more common with men, sexual desires are strong and tempting. When another beautiful man or woman offer themselves for the taking for just a night and nothing more, temptation takes its course. Most men or women who cheat just want to satisfy their sexual desires without any personal intent of feelings or attachment.
5. They are douchebags
There are some people who just cannot keep it in their pants and have no real intention on staying with you for the long run. Some people do it for malicious reasons, such as revenge or to hurt their significant other. I honestly hope you never meet this person.