I'm not sure what causes people to lie exactly. Mostly, I think they are simply not happy with who they are. Perhaps they feel like shameful under achievers. There is something very sad about that.
Odd how many Navy Seals, combat vets, (mostly Vietnam) and black belt karate experts I came across in my 25 year law enforcement career. Mostly they are all gone and I am still here. Somehow, I survived.
I was not surprised to see some firefighter claim 9/11 hero status and get caught lying. Read how he minimizes what he said. Like I said earlier, pretty sad. http://news.yahoo.com/blogs/sideshow/fake-9-11-hero-admits-lie-fellow-firefighters-192558839.html