You have a pie, and you have to share part of it with someone else. The question is, how much should you offer? Logically, not much, and logically, the recipient should be happy to accept any amount, no matter how small, since it’s better than nothing. But here’s the rub. If that someone thinks you’re being stingy they might refuse your offer, in which case neither of you gets anything.
In the human version of this ‘ultimatum game’, where the pie is replaced with money, the players tend to make roughly equal or ‘fair’ offers, showing that human concepts of fairness can override the logic of economics. Exactly why this happens is uncertain. It may be that the donor is motivated by some internalised moral framework, or it may be that the donor feels nothing of the sort but is simply responding to the fear that the recipient is fundamentally irrational and will ruin everything by throwing their proverbial toys out of the pram in response to a perceived injustice.