I am a competitive latin dancer and with years of experience-- I still dance sort of cool rather than with lots of expression. It is hard for me to smile or express much when competing due to thinking about everything I am doing. However, this past comp. I had a breakthrough and smiled or expressed myself a lot more--I just chose to express myself more on the floor. Now when I dance socially I force my self to smile and look like I am having a good time. It is like developing a good habit.

It helps if you dance with a fun partner that's not too hung up on technique and everything being perfect.
A partner that I dance with over thinks technique and it shows in our dancing.
She is a good dancer, but at times takes all the fun out of it. Choosing fun partners will help you smile or look relaxed.

At some point one needs to do some mind control and smile or look like you are having fun while dancing. Everyone can choose and develop his own dance personality. There is a certain amount of theater or acting used in all dances.