There is a story of a CIA agent who was having difficulty building trust between himself and a tribal elder in Afghanistan. One day, he gifted the patriarch four Viagra, one for each of his young wives. When the CIA agent returned a few days later, a friendship had formed between them.

You should do right by others simply for the sake of doing right by them. However, reciprocity is also a powerful way to build relationships. When somebody does something kind for you, you feel obligated to do something kind for them in return. The Japanese have a word for this sense of obligation. It's called sumimasen and it literally translates to "this will not end".