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

Ask question asked 15 years, 1 month ago modified 9 years, 5 months ago 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, …

There is a subtle but important difference between the use of that and which in a sentence, …