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. According to merriam-webster:
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? 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.