I dont approach people, but when I sit there, I cross my arms, and keep the same face...usually nothing to be happy about, nor sad. So my facial expression never changes. I think people are intimadated by that.
I am not that some one that keeps a smile on my face all the time, I think that is fake, but I do light up and smile and laugh when talking to people. You may need to come out of your shell, try to smile, do not try to just blend in, put yourself out there, have faith in you, believe in yourself, if you do not, nobody else will.