Courtesy on one side never lasts long.

The proverb suggests that courtesy is hard to sustain when only one person or group makes the effort. Considerate behavior is more likely to continue when it is returned.

It can also serve as a warning: someone who remains polite despite repeated discourtesy may eventually stop. The point is the strain of an unequal exchange, not a duty to abandon good manners.

Use it to comment on a relationship or exchange in which one side consistently acts courteously while the other does not. It fits a warning that this imbalance may eventually exhaust the courteous party's patience.