Doing things I don't want to do

Do you ever go along with things because it's harder to think up an excuse not to? Is this not a ridiculous way to live? I say that if I really don't want to do something and it's not something that I am required to do(i.e. go to the doctor, etc.) then I should be able to say "NO" and not have to explain myself at all. I know that I tend to worry about what other people think about me and I've just about had it. Either I have true friends or I don't and if they don't want to hang out with me because I say "NO" once in a while then screw them! Ahhhh...I feel better now.