Why do people keep up images?

Why do so many people keep so many images in their head? I've got friends who have one image for their boyfriend, another for their best friend, another for large groups, another for when they drink, and yet another for when they are at home. Now which one is the real them? I used to think I knew them, but now how can you be sure? I think that it's time to stop catering to everyone's image as they want it seen, and to start hitting them in the face with reality.

It's becoming too hard to watch all these "people" change based on what they are trying to get out of a situation. I'm tired of dealing with new people every time I want to hang out with my "friends". If you aren't who you've been in the past, then you aren't welcome around me. My life is going to be too short to meet a new friend everyday instead of just seeing my old ones again. A new friend is a new friend, and an old one who can't figure out who they want to be is no friend, but a dead weight that will drag you down until you can't be sure who you are either.

So, still no reason why people keep up their numerous images, but at least a little understanding for myself about how I should deal with it.