Ten possible reasons people don’t answer your winsome grin with one of their own:

You have a piece of last night’s pork dinner stuck in your front tooth.

They’re porking your girlfriend and are afraid smiling will make them look guilty.

Your fly is unzipped, and they have mistaken you for a Level 2 sex offender.

They just swallowed a fly.

You traded your personality in exchange for personal success.

Their recent personality implantation surgery was unsuccessful.

Not enough Prozac.

Too much Prozac.

They find your constantly raging perkiness annoying.

They are annoyed by a raging case of perpetual constipation.