I figure it’s because I know that an event is going to take a certain amount of energy out of me. Energy is a finite resource that only gets topped off after I get enough sleep (I can’t take short naps either. I take forever to fall asleep.) Therefore, if I have something planned for the evening, I feel compelled to “reserve” my energy so I don’t feel too drained by the time the planned event happens.

By contrast, if something happens in the moment, I might have enough energy for it just by chance. I’m not concerned with “saving up” my effort if there’s nothing planned later, so I can go with the flow as long as I can keep pace.