How do you think a person becomes/ends up being pretty much a complete asshole?

There seem to be assholes who were raised by assholes, no surprise there, but there also seem to be assholes who appear to have been raised by perfectly nice people and perfectly nice people who appear to have been raised by assholes. What do you think the “etiology” of assholery or assholeness? A lot of research has been done looking into how people end up becoming criminals, but there doesn’t seem to be much research on the causes of good old fashioned everyday assholeness.