There are many people who are smiling when you make an eye contact with them. One particular difference I have noticed among them is the shine of their eyes. Some eyes are bright and glittering while others are dull.
My theory is that the difference comes from the smile mileage. Just like a car mileage: the amount of time one has smiled in the past.
Those who habitually smile have more shine in their eyes and their expressions seem to be more genuine. They also look happier during all circumstances. While those who do it only for a specific moment tend to appear fake. Simply said, more time spent on smiling creates a deeper change inside.