kindness begets kindness; you scratch my back and I'll scratch yours
Proverb meaning that if you show goodwill to others, they will respond in kind. Literally: 'if a fish is kind to the water, the water will be kind to the fish.'
See also: 魚心あれば水心
They say kindness begets kindness, so let's try being kind first.
A traditional Japanese proverb. The exact origin is uncertain, but it uses the metaphor of a fish and water to express mutual goodwill.