Good explanation of why it's optional in this case, although i'm not convinced that reason is the only reasonable antecedent of why. For example, the explanation why is a common usage,. Why is used here as an interjection.
Understanding the Context
According to merriam-webster: —used to express mild surprise, hesitation, approval, disapproval, or impatience 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. This seems the simplest and most elegant expression of the meaning.
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Key Insights
I am always suspicious of reason (s) and why being next to. I don't know why, but it seems to me that bob would sound a bit strange if he said, why is it that you have to get going? in that situation.
