Start at the beginning. Why are they fake? Are they hiding something? Are they trying to be something they are not? Fake people are usually fake to get something they don't have. Other people don't matter in this perspective, only their goal matters. It is being selfish.
Ego and selfishness. There will always be and always has been people of this sort. This is basically your bullies from school. It is probably a lack of self worth that they must treat people this way in order to make themselves feel superior. These type of people need to be avoided.
I agree with the first two answers, these type of people have their own problems and are selfish, inconsiderate, and toxic to be around. They don't even like themselves, they have life issues and try to cover it up, or not think about it by being hurtful to others. I know a person like this...others do what she wants and don't try to say anything to upset her, because if they do, she will let lose. Very manipulative, better to not be around this type of person. Of course, when this type of person, is around someone they are trying to impress, or get close to...they can be so nice; but, like you said, it's fake. They are doing it for a reason, to get something they want.
Some may be perceived as fake due to lacking empathy for others. This could also be due to Mental Health issues. We need to stop judging others so quickly and try to identify the root of the behavior. Sometimes there is such pain in their lives they know of no other coping skills for negative emotions. I tend to follow the belief by Randall Terry,
Fool me once, shame on you. Fool me twice, shame on me.