I Want Junk Food

Why is it that I always want junk food over healthy food? Is there some sort of gene inside me that I am missing? If I am handed healthy food, I will eat it and generally like it but when I have to choose what to eat, I want pizza, chips, fried foods, hamburgers, pasta, etc… Help!! How do I change this vicious cycle?

Just in case you wondered, that pizza was so delicious!!