Reciprocity, the expectation that a favour or kindness will eventually be returned in some form, is one of the more universal social norms observed across human societies, functioning as a kind of quiet social glue that keeps ongoing cooperation between people worthwhile over the long run.
The norm does not require exact, immediate repayment, which distinguishes it from a simple transaction: a favour returned weeks later, or passed along to someone else entirely, still tends to satisfy the underlying expectation, which is part of what allows generosity to circulate loosely through a community rather than being tracked precisely.
Communities where reciprocity is strongly and reliably observed tend to sustain higher levels of everyday cooperation, since people are more willing to help a stranger or a neighbour when they trust that the broader norm of eventual return, in whatever form, genuinely holds across the group as a whole. None of this needs to be dramatic to be genuinely worthwhile over the long run.