I feel the same way when playing or singing in front of my family. I'm no phsycologist or therapist, but i see it as being this (well atleast with me): With your family if you play something they don't like or if they don't recieve what your playing well, you got that anxiety in the back of your mind that they can always tease you or mess with you about that, but with a perfect stranger you only gotta to see them like once and if you mess up or they don't like what you play or sing, so what!! you ain't got them around to keep berating you or bringing it up. I dunno, but i kinda see it like that with myself.