Do you ever look at someone and think "They can't be that happy all the time?" "I bet it's all an act" you tell yourself. Yes, I realize there are people in this world that are generally very happy and only see the glass as half full...I say good for them.
However, many people try to fit in with a society that values beauty and fame quite highly. Often times, being yourself is not enough, no matter how happy you really are. They create the illusion of happiness because they have a need to appear as perfect or happy to the outside world to gain acceptance. They may have many personal problems but refuse to put on anything but a happy face.
Finally, there are the people who others think of one way but are actually quite the opposite. Recently a friend of mine made the comment that I was good with my money. While this is true as of recently, I didn't used to be smart with it at all. Because I never really talked about my financial woes with anyone but my closest friends this friend automatically assumed I was Suze Orman. Without even trying part of myself was made into an illusion.
Whatever the reason for one to create their illusion, if it makes them happy and doesn't harm anyone else, I say go for it.