What do people do when they create viruses?
A person has to write the code, test it to make sure it spreads properly and then release the virus. A person also designs the virus's attack phase, whether it is a silly message or destruction of a hard disk.
So why do people do it?
There are at least three reasons. The first is the same psychology that drives people to set fire with intention. The second reason has to do with the passion of watching things blow up. Many people have a fascination with things like explosions and car crashes. The third reason probably involves showing off rights.