Nurture: I tend to think that how you eat as a kid greatly influences how you eat as an adult. If your parents overcook vegetables or feed you frozen, rubbery veggies as a kid, of course you're going to hate vegetables, because they tasted gross. And if they served simple foods, you may that stuff forever and not be willing to try foods that are 'too gourmet' or ethnic.
Nature: But then I'm sure there are kids who think their parents' food was mediocre at best and endured it until he/she one day fulfilled his Chowhoundian destiny.
On the flip-side of the 'nature' coin, perhaps people who are picky eaters were born w/ overly receptive taste buds, and flavorful food is an attack on their tongue; too much to handle.