It has different meanings.

• Some people assume you don't mind and are just interested in how you would look smiling.

• Some people think you're sad and by forcing a smile it would lighten your mood.

• Others think it looks better and everyone should smile all the time to highten the overall mood.

• Some think smiling helps people being successful and they are not entirely wrong. It can influence people you're dealing with.

... but of course they are giving it very little though how that fits with your current situation and maybe they can't even know it. So yeah, it's kinda creepy, lacking self-awareness and may be annoying, but at least most of them don't mean any harm.