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 it that everybody wants to help me whenever i need someone's help?
Understanding the Context
Why does everybody want to help me whenever i need someone's help? Can you please explain to me. 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 have flunked the exam, that is why i am attending coaching classes. I have flunked the exam, which is why i am attending coaching classes. Is there any change in the meaning of sentence. Why is used here as an interjection.
Final Thoughts
According to merriam-webster: —used to express mild surprise, hesitation, approval, disapproval, or impatience