Thus we say: You never know, which is why but you never know. That is why and goes on to explain: There is a subtle but important difference between the use of that and which in a sentence, … Why is used here as an interjection. According to merriam-webster: €used to express mild surprise, hesitation, approval, disapproval, or impatience The grass is wet because it rained last night.

Understanding the Context

This seems the simplest and most elegant expression of the meaning. I am always suspicious of reason (s) and why being next to each other. There can …