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 each other.

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 sentence, Ò€¦ Why is used here as an interjection. According to merriam-webster: €used to express mild surprise, hesitation, approval, disapproval, or impatience

Why is used here as an interjection. According to merriam-webster: €used to express mild surprise, hesitation, approval, disapproval, or impatience