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,.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, and i don't.I don't know why, but it seems to me that bob would sound a bit strange if he said, why is it that you have to get going? in that situation.