Most children ask why over and over until they get told, "I don't know", "you're done", or "because I said so", or some other exasperated answer. I used to tell my son that I have already answered his question, so he doesn't need to ask the same question again. This kept the questions to a minimum, if that is possible.
I watch another little boy in my house, who is two. He started asking why a couple of weeks ago. It's innocent enough. Pretty standard. Then, overnight it seemed, his why asking got out of control. It was a like a virus invaded and took over his speech. Everything out of his mouth is "Why?" Here are some examples:
Me: Time to eat.
Him: Why?
Me: Let's read a story.
Him: Why?
Me: Let's put on your shoes, your mom will be here soon.
Him: Why?
(Seriously? Didn't I already answer that?)
I know it's a phase, but he's got it bad. But, as childhood goes, when the why phase is over, another will replace it. And, when his why phase is over, my own daughter's won't be too far behind.