I just finished the book The Righteous Mind by Jonathan Haidt and was struck by a line at the beginning of his acknowledgment section. Relaying what a graduate student once taught him he explains that we do not express gratitude to settle debts or sow the ground for favors. We give our gratitude out to make stronger relationships.
I was struck by what a simple, yet profound thought that was. Gratefulness is not a selfish tool we wield to get the upper hand. Humans, while highly rational about a great many things, operate more like bees in a hive than most would give us credit for. Hives need cohesion. Strong relationships breed cohesion. The process is so clear.
Do you imagine that chimps can express their gratitude to one another?