Why Can't I Be More Like Me When I'm Around You?

I realize that when I'm around my boyfriend i completely change who I am... and I can't say that I like it. I calculate just about every movement, I change myself so much that sometimes when a great oppurtunity comes by for me to be my sarcastic, weird self i just clamp up an keep my mouth closed... I guess i'm still trying to get comfortable with myself in this relationship. It's pretty hard, and some people even critisize me for it. I don't know, I just want to know why i can't act like myself around him.