Actually, I want to talk about a different kind of "why?" My daughter asked me a little while ago what I think the most important parenting advice is. This wasn't a discussion about loving your kids, or supporting their dreams, it was more along the lines of day to day parenting. I told her I thought it was extremely important to help our children understand why they are told to behave in certain ways. I gave her the example of taking a young child out to a restaurant. We tell them to sit still, don't throw your food on the floor, be quiet. Those are all important things to teach them when eating in a restaurant. And, if we enforce it, they will learn to behave that way, probably because they know they'll get in trouble if they don't.
Yet, I think it's better to say, "You need to sit still because when you squirm around, it makes the people sitting with you uncomfortable, and we want them to enjoy eating with you." "Don't throw your food on the floor because then the poor restaurant workers have to clean up after you, and it isn't nice to make them have to do that." "Keep your voice quiet, because when you're loud, all the other people eating at this place are disturbed, and we want them to have a nice meal too." When that message is reinforced, your child will behave properly, not because he'll get in trouble if he doesn't, but because he's learned to be considerate of others.
Continue this into the teenage years, and instead of "Don't drink. Don't take drugs. Don't sleep around, or you'll get arrested or an STD or pregnant," you'll have a message of "What kind of person do you want to be? Look around you at the people who drink and take drugs. What does their future look like? What would it be like if you had a baby in high school or while you were preparing for your future at college?" Here's another powerful question - "Think about the kind of person you want to marry and be the father/mother of your children. Are you the kind of person he or she would pick?"
"Why?" can be a very powerful question.