Lots and Lots of people look toward others on who they want to be. They see how people around them affect their lives on who they will be in the future.
It's is not a bad thing to find a role model and grow a habit of doing something they did, but copying someone else's way of being is not yourself, and showing it to the people your trust and care about can be seen in a way as betraying, lying to them.
I've seen people act that way with me, I've noticed and it hurt because it felt like I wasn't fully understanding them, I wasn't being a proper friend that I had wanted to be.
But then again you can see where some people are coming from. Sometimes they want to be someone else to hide from something. That 'something' is a broad category, it can be the fact that if they reveal who they really are, we'd hate/mock/harass them. They'd want to avoid that under all circumstances. I can undestand the pain they go through, but it's appreciative mostly when you are yourself, people can trust you, they know you. The feeling of no having known someone unlike the fake exterior they kept up, to me is like I'm talking to a vessel that I only know the shell of but not the inside, which to me would just be hollow.
Like some people said, "The eyes are the windows to one's soul." I couldn't agree more, that's why I've learned to read people through their eyes like a book. Most of the time it is obvious because lying take effort, any slight shift in the eyes or hesitation with body language is like a dead giveaway (unless they're talking about something quite embarassing). My dad taught me this and it's quite useful because we move a lot. Read someone not by the words they say but the way they move. I've watched and learned the way one moves when one feels agitated, happy, sad, mad, etc., but I've met some people in my life that lie like they eat food. It's practically automatic. I find this depressing and pitiful. On the base that they'd lie for another one's safety is fine, I'd do the same. But lying so much that you barely even have to think about it, say it so naturally is sounds like your asking about the weather or the time, is shameful.
With that the trust built up between a friend and you deteriorates so quickly you'd find yourself wanting to stay away from them whenever they asked for something at the drop of a hat, tire once they say something that could be as small as "I went to the riverwalk" turn out to be a silly lie. Where can trust be formed there?