"Be still and know that I am God." --Holy Bible
We walk around all day encased in our thoughts, having a good day or a bad day depending on how we feel at the moment.  But what does it really matter to the person you pass by on the street, in the store, or in the hallway?  Does anyone care what kind of day you're having?  Why yes, your mood is affecting everyone you pass by.  So perhaps, you don't know each other, or maybe you do.   But you affect everyone and everyone affects you. 
If you walk around with a frown, that transfers - think of emotions like water, or air.  You swim in them, breathe in them, are completely immersed in them.  They affect how you feel. 
So what is the affect that you want to have on the world today?  Do you want to make it a happier place?  Then try smiling when you pass by strangers.  They'll see the twinkle in your eyes, the glow in your face, and they might just smile back at you.