You Never Know

I was thinking about the phrase "You never know," and I was wondering how realistic it was. I mean we use a lot of expressions that even have opposite expressions, like "He who hesitates is lost," and "Look before you leap." And then we have some whose meanings are more obscure than others, like "A bird in the hand is worth two in the bush." We basically get it, but it's not a good obvious one like "Don't count your chickens before they hatch."

So back to "You never know." I was thinking is that really applicable in reality? So to test it, I tried the opposite approach of "You always know ______ ." And to be perfectly honest, all I could come up with (besides jokes like "half the new network shows will suck every Fall,") were "You're going to die," and "You have to pay taxes." So the expression "Nothing is certain but death and taxes" rings true. So if those are the only things that you can apply "You always know" to then I guess you can apply "You never know" to everything else. But, you never know.