Well its animal instinct. Humans have become self conscious but their subconscious mind always follows 'cave man principle'. So when people don't have something true to show they just pretend to have. And usually these people are weak in every term. Understand them and give them time, they are just trying to survive in this harsh world. point out their fake self and they will respect you for your truth and eventually you will realize that they are really good folks
From what I've seen, mainly that last one.
its called acting, ever heard of it?
all the world is a stage and we are nothing but actors, Shakespeare
Denial, not familiar with themselves
take it is a fear within them, that they don't even realize exist.
usually people that are unhappy with who they are, or are in need to be accepted by others that do this..