People who volunteer or help a stranger often describe a distinct lift afterwards, sometimes called the helper's high. It shares some of the same biological signature as other rewarding experiences: a brief change in mood and energy that follows shortly after the act itself, not before it.
Researchers studying prosocial behaviour have connected this feeling to the body's own reward systems, the same general pathways involved in other pleasurable experiences. Giving help appears to activate them in a way that receiving help, while valuable, does not quite replicate on its own, at least not as reliably.
What makes this useful beyond curiosity is how ordinary the trigger can be. Holding a door, carrying a bag, or simply listening properly seems enough to produce a version of the effect, which suggests the reward is not reserved for grand gestures but built into small, repeatable ones that fit easily into an ordinary day, long before anyone would think to call them generous.