Here's why:
* Shaving only cuts the hair at the surface of the skin. It doesn't affect the hair follicle, which is the living part responsible for hair growth.
* The appearance of thicker hair is due to the blunt end. When hair is shaved, it's cut off with a blunt edge. This blunt end can feel coarser or more stubbly as it grows back. Also, hair that hasn't been exposed to the sun and elements can appear darker.
* Hair naturally tapers. Hair that grows naturally (without shaving) has a tapered end, which feels softer and finer.
So, while it might seem like your mustache is thicker after the first shave, it's just the illusion created by the blunt cut and the initial growth of previously untouched hair. The actual thickness and growth rate of your hair are determined by genetics and hormones.