One ungrateful man does an injury to all who stand in need of aid.

When help is repaid with nothing, the helper becomes a little slower to help the next person, and the next person had no part in it. Generosity is partly a habit and partly a bet, and a bad outcome makes the bet feel foolish. The cost lands on people who will never know why the door was less open than it used to be.

If someone has helped you, say so specifically and in a way they can hear. It is not about squaring the account. It keeps that person willing, and the willingness is what somebody else will need later.