People Livin' In Competition/ All I want is to have my peace of mind.

Why do people feel the need to cheat? Much of that, I believe, can be attributed to the focus for everyone to be the best. I believe this to be true because of the pressure to graduate with straight A's or as Valedictorian. What happens when someone notices several people with the same report? THIS!
That same link also shows how it can affect someone outside of school too. In addition to that here is one from a Psychiatrist, who could potentially lose his business across the pond.
Plagiarism, however, is rather easy to avoid. Here is a nice safe guide from Purdue University. Which basically states that you just need to give credit where credit is due. Even with easy guides as such. I think Mr. Clark has a nice idea in his article hear, where he basically calls for a better more universal definition.
I chose the title of this (of course coming from the band Boston) because I think that's why people plagiarize, they're trying to compete. However, if one should just do their best and take the peace of mind that they know they won't get into trouble, in the end it will work out better. My High School English teacher told us the story of one of her husband's friends:

This man lied about having a degree and was hired by a company, that didn't check,when they asked him to go to school, that the company would pay for, to get his master's degree...he had to come clean.Obviously he lost his job.

It will get you in the end. All you really need to do is have enough self-respect to trust your own thoughts, ideas, and research; and, when you need someone else's opinion, you just need to make sure you give credit where credit is due.