why? WHY does my daughter say why all the time?

Depending on her age, she might be: Repeating a new word she learned. Genuinely curious. Just trying to annoy you. Since you mentioned changing diapers, I would guess she falls into the first or maybe second category.

LOL she's at what I refer to as "the sponge" stage. They want to learn everything and they do this by asking "why". I'll never forget my oldest son was under 3 when he went through this, all day long it was why why why. I was getting ready to go out with my husband that night and I had after all day been trying to give "good answers", he asked why as I was putting on my make-up and I answered, "Just because". I never did that again, he looked at me as serious as could be and responded, "That's not a berry good weason, Mommy"...