A smile can hide a wide range of emotions. Sometimes we smile to hide our sadness, sometimes our anger, indifference, annoyance etc. But you can usually tell if a smile is genuine by looking into the eyes.

One of my teachers once said: „A smile is the most polite way of baring one's teeth." ;)

Sometimes I hide fear and suffer also disappointment. Than I have to pay back sometimes, because when people think you don't know that they are using you they begin to laugh at you. I smile for myself and go forward.

Thank you guys for all of your meaningful comments °˖✧◝(⁰▿⁰)◜✧˖°

Smiling is attitude to life in general or to a particular situation.

But you are right. Smiles help to hide your annoyance with a person, especially with someone you aren't likely to see again) It can work to the contrary with close people as any misunderstanding and conflicts are more easily solved at the earliest stage...

What else?

Indifference. Perhaps, it's the most frequent case.