Why is used here as an interjection. According to merriam-webster: โ€”used to express mild surprise, hesitation, approval, disapproval, or impatience

This is a common english phrase that i'm sure everyone has heard before. However, i find it. 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. The grass is wet because it rained last night.

That is why and goes on to explain: There is a subtle but important difference between the use of that and which in a. 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.