Be kind to Everyone
You never know what other people are going through. Everyone has challenges in their life. Everyone faces different hardships or struggles that they're working on.
One of the keys to happiness is to be kind to everyone you see. You never know when a kind word or smile can transform someone's day or week, and make your life a whole lot better in the process.
So smile. It's free and usually makes you a lot happier than you were if you didn't.