People cheat because they are greedy. They are not content to have the relationship, they want more as well. This is why cheating is so universally condemned. It's a sign of bad character and bad choices. It is never acceptable, and those who cheat once will usually cheat again.
Lack of communication is a huge downfall for many couples. One partner may be extremely unhappy, but instead of sitting down and respectfully voicing their opinions, concerns, and feelings, they hold things in until so many issues build up one upon another. They may become 'emotionally separated' from their partner before actual physical cheating takes place. The unhappy partner is also known to create a fantasy world in another person, and sees everything in this person they wish they had in their actual partners.
I think there are any number of reasons why people will cheat, but one of them is a lack of understanding about what it means to be committed to someone for more than just the first little while when it is exciting. The idea that people fall "out of love" is confused with the fact that people aren't willing to deal with the tough things that will come up in any relationship that can be solved with work and nothing else.
There are many reasons why people in relationships cheat on each other. It varies widely with the individuals involved.
For example, when a couple is young, they may cheat on one another out of insecurity. The members of a young couple may want to prove to themselves that they are desireable. They may worry that only their partner will find them attractive and they may want to go out and reassure themselves that they can be attractive to other people.
As another example, members of an older couple may cheat out of boredom. They may fell that their lives have gotten uninteresting now that they have been together for so many years. Being unfaithful, in that context, might provide a thrill that has been difficult to find for them in their own life together.
These are two of the many reasons why people might be unfaithful to their partners.