I've had a conceptualization lately of want as an electrical circuit: it's either open or closed. On or off. The phrase "I don't know what I want" strikes me as absurd. When you want (or don't want) something, you'll know it. Want is positive or negative, but not neutral.
"I don't know what I want for dinner" means you can't decide what to order from the menu. There's a big difference between indecision and want. I want the steak. I don't want the deep-fried parmesan [gag]. Don't say "I don't know what I want to order," but rather, "I can't decide what to order."
I tried explaining this to my brother a few weeks ago, and he asked if I had been smoking pot.