You shouldn't have to they know you are what they're trying to do is change the way you think and behave simply because they feel alienated from you because they never had the chance as kids to express themselves so openly and I think they feel that you should do as they did as kids. Hey, parents are parents and they do believe in you it's just sometimes they have difficulty in showing it. :)
Let them know who you are by the actions you take.
There are some things that no matter how much you talk or what you say will change people's mind. Parents are a good example. But, like it or not, there is an effect for every action you take in life. Make sure you take action that gets your goal reached instead of some sort of penalty. Keep in mind that you are your parent's responsibility and they have power over you while you are their responsibility. I'm not taking their side, I'm just stating a reality.
You don't have to be head cheerleader, first string quarterback on the football team or wear pink petticoats if that's not who you are. But be who you are and don't be anything just to conform to some group, whether it's goth or school prayer group leader. I would say the same thing to the head cheerleader. Be your own person.