Once upon a time, man asked God for expression. 
So God made the eyes windows to his soul. 

As it always goes, man asked for more. 
This time he wanted a didactic device. 
An expression of pain/hurt/sadness, as well as a means to relieve the same. 
Thus, God bestowed upon him tears. 

Nevertheless, some were different from the others and they did not want to express/reveal their unhappiness each time. 
One such asked for a mask, a disguise of sorts. 
And so, a smile came into being.