Reciprocity is a trait that we have been taught since we were infants. It is the ever old saying about Karma. You receive what you give. If you display a certain trait towards someone or something, chances are high that you receive it in equal or higher measures.
All you have to do is to look at your canine friend. Dogs are experts at reciprocating anything given to them. We often talk about the loyalty that dogs exhibit. When you adopt, shelter and feed a dog, they automatically get trained to display loyalty.
Reciprocity is an innate psychological characteristic. In fact, scientists and psychologists have hypothesized that reciprocity is a basic survival mechanism. Within the animal kingdom, species coexist and support each other to ensure their survival. The natural impulse in us to give back is the reason why this is the case.
Forever, humans have shared their food and talents with others around them, expecting that this results in an obligation for the other party to reciprocate. Such an obligation results in a web of interdependency that can be exploited by the helper. It is a proven psychological trait.
Families also operate on a similar logic. When a lioness births, feeds and raises a cub, she creates a strong impulse in the cub to reciprocate. It is the reason why societies operate the way they do today, with children taking care of their parents whey parents are older and need assistance. There is no other legal framework or obligation in place that can explain this behavior.
This also helps in explaining the supreme loyalty present in dogs raised in a positive manner. We often marvel at the way a dog can jump into the face of danger to help its owner, despite an inherent risk to its life. It is reciprocity at its highest pinnacle.