You never know what kind of day someone's having.
I was in the grocery store when a man gave me a nervous smile. I noticed my cart was blocking his way and I quickly said, "I am so sorry, let me move this. After you brother."
Suddenly, his nervous smile was gone. He seemed surprised and said, "I've been having a really bad day and you're the first person to be nice to me. Thank you."
I thought about it for a second and responded, "Wow. Thank you! You're actually the first person to be nice to me too."