perception of people can be so warped

its amazing at how in the matter a of few seconds someone can completely change how they treat you or who they are.
at this point I am not even sure if i am able to understand why it is that people are so afraid to show who they truly are. is it about personal gain, drama, being nosey, or just being selfish? I wonder how many people are able to see through the lies or able to see other parts of your personality that are always shown?
Why is it people only try to show their good sides in the beginning of a relationship (whether it is a frienship or dating) and pretend to be someone else? It can be confusing to meet a person who comes across as beign independent, honest, trust-worthy, and having all of these wonderful qualities. Only to find out later, when this person shows a completely different side containing things such as insecurities(personal or with other people), immaturity, dishonest, and so on. Then this person usually feels its necessary to play games, eiher to test that person or to make themselves feel better. I have never claimed to be perfect and I can admit I have my own flaws, but I wont hide from them and try to stay true to myself.
I led myself to believe false assumptions, thinking that you were someone that you werent even close to being. Now I am left wondering who you really are? And the person I see today, is that really you or is it fake?