Look happy as people are attracted to happiness but repelled by misery.
This expression was coined by American poet Ella Wheeler Wilcox (1850-1919) in her poem "Solitude" (1883):
Laugh, and the world laughs with you;
Weep, and you weep alone.
For the sad old earth must borrow it's mirth,
But has trouble enough of its own.
Sing, and the hills will answer;
Sigh, it is lost on the air.
The echoes bound to a joyful sound,
But shrink from voicing care.
Rejoice, and men will seek you;
Grieve, and they turn and go.
They want full measure of all your pleasure,
But they do not need your woe.
Be glad, and your friends are many;
Be sad, and you lose them all.
There are none to decline your nectared wine,
But alone you must drink life's gall.
Feast, and your halls are crowded;
Fast, and the world goes by.
Succeed and give, and it helps you live,
But no man can help you die.
There is room in the halls of pleasure
For a long and lordly train,
But one by one we must all file on
Through the narrow aisles of pain.
But the essence of the idea can be found in the work of Roman poet Horace (65-8 BC): Ut ridentibus arrident, ita flentibus adsunt humani voltus (Human faces laugh seeing those who laugh, and correspondingly cry seeing those who cry)