Like attracts like.

物以类聚。

No sunshine but bath some shadow.

有明必有暗。

Nothing for nothing and very little for a half penny.

一分钱，一分货。

Speech is silver,silence is gold.

言语是银，沉默是金。

Take a pain for a pleasure all wise men can.

智者皆能视苦为乐。

Thoroughly to teach another is the best way to learn for yourself.

教学相长。