A positive sum game

My family went to Dalat for vacation. We went to Dalat market for lunches everyday. The first day, we went to a very crowded restaurant. Waiting for the available seats, another restaurant's owner, who was selling soft-drinks, waved and offered us the seats. We got lunch and did not forget to reward her some drinks. We went to another restaurant the next day. The owner was so busy with many customers while her neighbor did not have anyone. The neighbor went to the busy one. She washed dishes, cleaned tables and delivered meals. When I asked, "why do you work for your competitor?" With a smile, she said, "Tomorrow, if I am busy, my neighbor may help me."