There are lots of reasons why people cheat. There's the really blunt answer in that people just want something different, something new (just to try it). I think, when people are young, they want to have their cake and to eat it as someone already said. They want the security of a relationship, but also want to keep exploring sexually. People can be incredibly selfish when they want to be, not to mention cruel.
For example, if you get into a relationship when young, you may really like that person, but also be curious about what else is out there (the grass is always greener it seems). Do you want to stay with the first person you have had sex with, or do you want to have your fun when you're young? If you decide on the latter, you have to break off what was a solid relationship or cheat. Often, people don't know what to do in that situation. They think that cheating is somehow justifiable as an outlet for their problems, when really it is a reflection of them. Frankly, in that regard, people are selfish and lack respect.
Then you have the sort of infidelity that marriages go through. This is when one partner gets bored, feels unappreciated, wants to get revenge etc. over a long period of time. They may feel trapped by children, mortgages and so forth, but also emotionally/physically neglected. Any marriage counselling service will tell you that the cheating is merely symptomatic of a breakdown elsewhere, be it in communication or intimacy etc..
You can probably divide up cheating into two broad categories as to whether it is largely premeditated, or whether it happens on the spur of the moment. In the premeditated section, you have the person actively seeking out new sexual partners knwoing full well the consequences. In the spur of the moment category, you have drunkenness and impulse.
I actually feel that the two types reflect different problems. The first category stems from more holistic problems in the relationship where one party is unhappy (whether justifiably so or not) and decides to remedy that with cheating. They may be bored sexually, or regret their relationship but not have the courage to end it. This is usually in more long term relationships where there is far more at stake.
The second category reflects people being selfish/foolhardy/impulsive. This is where I have seen a lot of damage done to young relationships. The relationship in question may be fine, but the person in question feels that the risk is worth it. I feel that this is all down to maturity and age. Maybe also a lot of young people are in relationships simply to avoid social stigma and really were after sex the whole time. In that case, cheating is largely immaterial to them.
I am certainly not saying that cheating on impulse is justifiable or any less damaging. It is more that the relationship or the cheated upon party is not to blame ... the cheater is just weak.
I guess the final point is that cheating as a mating strategy is about game theory. It was summed up nicely earlier on, and all I would add is that cheaters feel that the reward is worth the risk.