My take on this one is simple: boredom. Once that "first flush" has gone, and reality sets in, some people really just cannot cope with it. They realise that their partner has faults and that real life is full of real struggles, good times and bad.
Of course, it can be more complex than that: there are other situations that can drive people to cheat, such as domestic violence or lack of support within the relationship, not to mention life-changing events such as death within the family.
Then you have the people who cheat just because they can. These people are incapable of holding down a monogamous relationship and this scenario is common in people who have seen similar behavioural patterns in their own parents.
I personally would choose not to cheat, rather I would encourage people to work on the relationship they have and discuss issues. If they don't do that, then the problems will follow them through every relationship they ever have.
I hope this helps!