He who has been kind to another and then finds that his kindness has been thrown away passes the remark, A camel only makes water from behind.
-Wit and Wisdom in Morocco, Edvard Westermarck, 1930
There is something in this proverb that speaks of pissing away a good turn done to another, but it also suggests a different reading, that you shouldn’t look left or right when doing what you’d rather leave in the past.