Thus we say: You never know, which is why but you never know. That is why and goes on to explain:

Where does the use of why as an interjection come from? This is a common english phrase that i'm sure everyone has heard before. However, i find it. The grass is wet because it rained last night. This seems the simplest and most elegant expression of the meaning. I am always suspicious of reason (s) and why being next to. Why is used here as an interjection.

This seems the simplest and most elegant expression of the meaning. I am always suspicious of reason (s) and why being next to. Why is used here as an interjection. According to merriam-webster: β€”used to express mild surprise, hesitation, approval, disapproval, or impatience