Here is a possible short story ending with why it pays to be good to strangers:
Jake was having a miserable day. He woke up late and missed his bus, causing him to be nearly an hour late for work. His coffee spilled all over his shirt, and in his rush to get changed he cut himself shaving.
Feeling frustrated, Jake took a seat on the bus. An elderly woman smiling brightly sat down next to him. "Don't look so down, dearie," she said brightly. "The day is only getting started, things will get better!"
Jake grumbled in response and gazed out the window. But her optimism began to lift his mood. By the time he arrived at work, he felt rejuvenated.
At lunch, Jake went to his usual sandwich shop. But the cook was out, so the line was long and moved slowly. The elderly woman came in and spotted Jake at the back of the line. "My, my, not your lucky day is it dearie?" she said with a chuckle.
Jake sighed and shrugged. To his surprise, she walked up to the counter and placed an order for sandwiches for the whole line. "My treat, since you look like you could use it," she told Jake with a smile.
Several people in line thanked her profusely. Jake shook his head, "But why? I didn't even know your name..."
"Names are unimportant when it comes to kindness," she replied. "Pay it forward someday, and spread the good cheer."
She was gone in an instant, like a bright light passing by. Jake smiled, realizing she was right. Being good to strangers paid off in more ways than one. His day had started out poorly but ended up bright. Her generosity and optimism had touched him deeply. From that day on, Jake always made a point of being kind to strangers whenever he could.