If it makes you stop taking things for granted, it's probably a great technique. But if you ask why and you are satisfied with a boring answer you won't get much out of it. It's more about the state of mind it creates than what particular thing you are doing. But in general, I'd think that if it makes you think about ordinary, "mundane" things, you will probably be likely to become lucid.

I never thought of this btw. I might start asking "why" now at random times, thanks