Why Is Junk Food So Good

What is it about junk food that makes it so appealing, so good? I always wonder about why we are attracted to eating things that are utterly bad for us. Why can't we be drooling over broccoli or brussel sprouts instead of pizza? I think that some smart genetic engineer should find the switch to turn off the cravings for bad foods and turn on cravings for healthy ones. Except chocolate. Nobody mess with my chocolate...