Why do I have a hard time smiling?

Whenever I'm around someone I'm close to, it comes natural, and I have nooo problem smiling. But then on cue, if someone is trying to be polite and smiles at me, I TRY to smile but it comes out really fake. This happens all the time, and it's really annoying because I wish I could smile REAL at least to most people, but it's like my face muscles lock up and my smile comes out so strained. Why is this?