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 should the first person pronoun 'i' always be capitalized?
I realize that at one time a lot of nouns in english were capitalized, but i can't understand the pattern of those.
Why is it that everybody wants to help me whenever i need someone's help?
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.
Is there any change in the meaning of sentence.