Virus creators are a diverse crowd but they fall in four main categories:
Complete idiots: (yes, programmers can be idiots) they like prank jokes and think that a farting microvirus is the coolest thing ever.
Self proclaimed anarchists: they basically hate Microsoft and think that Bill Gates is going to take over the world, their goal is to demonstrate that Microsoft products are crap.
Evil geniuses: they do it for the thrill, the challenge, their goal is to break the most secured systems in the world just to prove to themselves that they are the most intelligent people on the planet.
Hitmen: they do it for the money, they are paid by a company to shut down the system of a competitor.
But the main reason why viruses exist is because they have a body to infect: the Windows operating system. An OS with hundreds of security holes that no one at Microsoft seem in a hurry to fix. As long as Microsoft won't make any efforts to secure its OS, viruses will spread like fire. Of course, one of the main problems is that virus creators are often also Windows programmers...
