This reminds me of a story that's also kind of related to /u/PerilousAll 's post. I was out with some friends at a bar. The waitress messed up my order, so i asked her politely to fix it. She apologized and brought me something new, but it was wrong again. You could see it in her eyes that she had a shitty day already and she was getting annoyed with me, which in turn annoyed me. I didn't do anything wrong, I just wanted what i ordered.
I didn't intend to tip her well, so i asked for change. She must have misread the bill that i gave her, because she gave me change for 50€ instead of the 10€ i gave her. I only realized it after we had already left. My friends all congratulated me on being paid to go to a bar, but i felt bad for her. That money would probably come out of her pocket at the end of the shift. When i went back in, she looked like she was ready to explode. "What now?" she said, half screaming.
Seeing her expression change from furious to smiling to holding back tears after i explained the situation was definitely worth the money. I could see the frustration of a horrible day melting away. She gave me a long hug and the most genuine thank you i've ever received.

Not only was i able to make someone else's shitty day a little bit less shitty, her reaction also totally made my day as well.