I just ran across an article ...
There are various possibilities for what motivates a hacker / virus author:
1) Money - they can easily harvest valuable, private information from vulnerable systems that can be exploited to steal from others
2) Spite - talented people with a misdirected animosity toward technology or toward specific companies can write stuff to wreak havoc.
3) Terrorism - Don't you think those behind terrorist activities have the interest, the funding, and the resources to continue to attempt rewinding history 300 years?
4) Boredom / entertainment - the morbid fascination with others' demise has never ceased to amaze me.
5) Social kudos - what hacker / nerd would be prouder than one that is able to show off the global, destructive effects of their own work to their cohorts?
Other possibilities are probably not just occurring to me at the moment.