As strange as it may sound I tend to think of my mind as run by a whole bunch of different people. Each persona is unique and usually useful for dealing with a particular situation.
There is dealing with authority figures Me for use around teachers. He is rather submissive, but firm.
There is social me which is the most dominant one when I'm with friends.
Intellectual me for doing homework or analysing lectures.
Frak-Everything Me that just wants to blow stuff up and tell everyone to go piss off.
There is geek me whose sole ambition in life to score 100% on the Geek test.
There is dominant me that wants everything to be the way I want it to be.
There is submissive me whose sole purpose in life it to fulfil the desires of another.
There is the me that no one has ever seen but me. He only exists when there are no others around and nothing with which to interact except my thoughts.
Of course there are others that exist, but it is impossible to convey me using only words.
It's not quite as clear cut in my mind as that list makes it out to be. I am all of these, but they are each part of me. They each exist for a purpose. Now there are several factors that determine which combination of these people control my actions or thoughts. Whose company I am in has the biggest effect I think.
I've noticed this shift in the personalities of others as well. Depending on who you are with, you change who you are in essence. Some people are very confident and funny when they are with their close friends, but introduce and outsider and poof! they become shy. Your actions, goals, and responses change with your company. Prime example is the content filter that is automatically applied in the presence of adults. Even the make-up of your thoughts changes with company. This is what I find most disturbing/interesting about the whole thing. The things I think and the way in which they are thunk actually changes! Depending on the people I am more apt to: doing stupid things; thinking deep thoughts; starting arguments; remaining silent; and buying food just to name a few. It worries me to some extent that I can be so fluid and dynamic. But at the same time it makes sense. People who can do this probably survive better human social structures because to a certain extent being different people is required. When you are in the position below someone it's probably not a good idea to be critical of them if your only reason is because of bitterness. Likewise if you are above somebody letting out all of your hopes and fears can be dangerous because they are very likely to try and replace you.
Perhaps it's just me that is like this, but next time you are with somebody and you enter different company watch to see if they change. We are who we are with.