Monkeys keep tabs on who owes them something, and you don't want to be in monkey-debt.
Sharing is caring, but you're lying if you say you don't expect something in return, and science knows it. Researchers looked at 32 studies of primates and human foragers, and the evidence points to the fact that when sharing food, we want something out of it. Ass, gas, or bananas. Nobody rides for free.