expression
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. The literal image is that if a fish is kind to the water, the water will be kind to the fish.
魚心あれば水心で、こちらが親切にすれば相手も親切にしてくれるものだ。
As the saying goes, kindness begets kindness; if you are kind to others, they will be kind to you in return.