What is the origin of the idiom "you never know" and when to use it?

The phrase "you never know" is an idiomatic expression that has been in use for a long time. Its origin is rooted in the acknowledgment of life's unpredictability and the idea that the future is often uncertain. This phrase is used to convey that one cannot be certain about the outcome of a situation, emphasizing the potential for various, often unexpected, possibilities.