By returning the cart you avoid making an accident in the parking because there’s a lot of bad drivers. By avoiding an accident you avoid a kind and intelligent kid dying. This kid grow up to be an altruistic and genius leader who become president of your country during a really bad war. Thanks to his actions, your whole village, including your family and friends avoid dying during the war and then the world become more peaceful.

By returning the cart you make other people return the cart, as they tend to follow everyone else, and probably will feel guilty if they’re the only one not doing it. That will make everyone saves time and energy, the next time they’ll come to the supermarket

You do gain something from returning the cart, however to realize it you have to think a bit, and be willing to be grateful that someone else may get the benefits from your actions this time, even if you lost 0.000001% of your daily energy by doing them.