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. 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.
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Key Insights
Is there any change in the meaning of sentence. 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.
Final Thoughts
According to merriam-webster: —used to express mild surprise, hesitation, approval, disapproval, or impatience