why do this happen~

they could steal many things through internet using the trojan virus and other virus,like your bank account,your user ID,and they could check your mail boxes,so after all they could steal other's money or their privacies,I guess thesea are the motivation of their behaviours

my computer also have virus , and it always come out in my computer windows. and it waste me a lot of time to kill it. and i still can't skill it. it disturb me a lot when it come out. and i found it always come out recently.

The basic reason is to fulfil one's desires, motivated by the ego and greed. Trojans are called so because they hide in the system and wait until a predetermined time. Then the programs act the way they were designed and meant to. Usually trojans cause damage to computer systems, and not necessarily the system that has become an unwitting host. Sometimes the programmers are only trying to prove a point. In many cases the actual damage is great when the time lost is taken into account. As long as the ego reigns supreme cases of trojan attacks will not cease. Many other negative attitudes and behaviour are related to ego. all the best, rosdimy

People spread viruses most likely because they created them. I guess it is like watching your kid growing up. Even though it will be horrible, and you know it, you will still love and watch over it.