Services unrequired go unrequited.
More German Proverbs:
A woman and a glass are always in danger.German
They play till they quarrel.
German
He who has no shoes dances in his socks.
German
The dog returns to its vomit.
German
Three, helping one another, bear the burden of six.
German
In the looking-glass we see our form, in wine the heart.
German