Happiness, sorrow, victory, defeat, day-night are the two sides of the me coin. Similarly life is full of moments of joy, pleasure, success and comfort punctuated by misery, defeat, failures and problems. There is no human being on Earth, strong, powerful, wise or rich, who has not experienced, struggle, suffering or failure.
Smiling is contagious. We all have our own battles to fight. Each one of us! And sometimes all a person needs is a smile to reassure them that everything will be OKAY (because it will be). Maybe one smile can reassure that girl who sits beside you in the class, that someone sees her - that someone actually cares. Maybe one smile to a stranger you walk by in town today is all it will take to turn that rainy cloud hanging over him into a bright shining sun.