The more I give to others the more I realize I have to give.

Giving works like an inventory count. You don't know what's on the shelves until somebody asks for something specific.

A friend needs a lift on a Saturday you'd mentally written off as full, and it turns out the Saturday held four spare hours that were going to be spent on nothing in particular. Someone asks a question you can answer in two minutes and you'd forgotten you knew it.

This isn't a case for saying yes to everything; people run themselves into the ground that way. It's that most estimates of what you have available are guesses made in a tired hour, and they run low.