It never rains but it pours, which is an idiom in China. It means that good things will not keep coming, but disasters will follow. From Shi Ming Nai 'an's "Water Margin" back to the 37th time: "After listening to this, Song Jiang decided that they would say," It's only bitter! It just never rains but it pours.
Happiness never comes in pairs, but it never rains alone. They are used as objects and clauses. Used in written language, can be used alone.