I am fine (insert fake smile)

"I am fine" is usually the most common yet mysterious lie we tell.  We are never fine we are simply covering up what we are feeling in order for that other person to get the news that ;"okay are you really okay or what is going on?" We say this because we hid the truth behind our fake smile in order to persevere our own status in life. Why is that we do this though? We are we covering things up so much? Why do we say "I am fine" when really we are not fine at all? There is something about this lie that remains to be a mystery in human nature that we have not yet discovered what it was, and yet we use it on a daily basis.