Why Cheat?

Cheating in a relationship is one concept that I just cannot seem to grasp. I don't understand how a person can pretend to care for someone and then go behind their back and screw around. Now, I suppose I should define "cheating" before I get to far into this, but as far as I'm concerned "cheating" is the participating in any activity that you wouldn't want your significant other to know about. This means that different relationships have different boundaries and while something may be cheating for one couple, it may not be for others. This is how you end up with swinger couples, or you have a person in a relationship that's a hugger and gets by with it because their partner doesn't think it's wrong to do so. The reason I have difficulty grasping the logic behind cheating on someone is that we each define the rules of our relationships, and if we're in a relationship we obviously care for the person we're with. If we didn't care for our partner we wouldn't be with them, right?
Apparently my logic goes above the heads of many people, and until recently I thought that infidelity was exclusive to youth. Working with older businessmen I found that many of them are quite open about their adulterous ways. It turns out that older people cheat just as much as younger people. I was told by a man that was once caught by his wife and nearly ruined his marriage that "having a mistress is far more trouble than it's worth". The reason it's more trouble than it's worth is that you're giving people a way to blackmail you when you cheat. People know that you're being unfaithful and they can hold that against you to get what they want. It could be a friend of yours, or a friend of one of your partners, or even the person you're cheating with that uses your flub as leverage to get something out of you. In the end you find yourself supporting two or more people financially, emotionally, or both, when you could be dedicating those resources to a single person.
There are several situations that are popular causes for adulterous activity.
-Having a partner leave you to enlist in the military may cause the person left behind to feel abandoned and therefor seek comfort. On the other hand, the miles between the new soldier and their significant other may provide them with a sense of freedom and invincibility. They may think that because they're so far from their partner there's no way that they could be caught cheating.
-Another possibility is that one member of the relationship might be a business person that travels frequently. This opens up the door to have several partners in different, frequently visited cities. If a businessman is visiting the same city once a month for several days at a time they may develop a new relationship in the area so that they don't have to give up on companionship while on the road.
-College is another situation that may lead to cheating. One member of the relationship may not make it into the same school, or they may not even try to get into the same school, but either way it puts distance between the two. Again, this distance creates a security blanket that may make one feel as though they can safely cheat.
-My final example is one that applies to pretty much anyone that's old enough to cheat in the most serious ways. The party scene. Parties are social events that encourage conversation, and when alcohol is involved people's inhibitions go out the window and things can get out of hand very quickly. Parties are not safe places for people that are trying to nurture a long term relationship as they often lead to unnecessary drama that puts a strain on even the strongest of relationships. If you MUST party while committed to a relationship it would be in your best interest to party with your partner present to reduce the chances of a screw up.
These are just a few of the situations that I frequently come across while lurking social media sites. There are many more ways, and reasons that it could happen. No matter what the reasoning behind the act it's still wrong. Often times the cheating leads to reciprocation because the victim feels the need to show the other what it's like. This is no way to go about mending a relationship. If you're going to forgive your partner because you truly think that they can change and never repeat their mistakes then you have to actually forgive them and move on with the relationship. If you can't forgive them for what they've done then don't waste any more of your time on the person. If you think you'll be able to forgive them after some time apart, then so be it, but constantly butting heads over it isn't going to undo what happened.
Keep in mind though, that if someone is willing to cheat on you once then they'll more than likely be willing to do it again, and if they'll cheat on someone else that they "love" with you, then they'll be willing to cheat on you too. Once a person has proven that they have it in them to commit adultery there's no way that you can be sure that they wont do it again. Be confident in yourself, and do what's right for you.
Again, I really don't see the point of cheating. It seems to me that if you no longer want to be in a committed relationship with someone that it's as simple as breaking up with them. Once you're single you're free to hug, kiss, make out with, and screw whoever or whatever you want because you are no longer attached to anyone else. I suppose it all comes down to human greed. People want to have a life partner, but they also want to have fun. Maybe that's why I can't grasp the concept of cheating... I would rather nurture my existing relationship and make it work than have one girl that understands me, and another one for intimacy. I guess maybe I just got lucky and got the total package... or maybe my brain works differently. Who knows.