Does this ever happen to you? Is it all in my head?

I'm barely ever hungry, I mean if food isn't in my face, I sometimes won't even eat for like 6 or 7 hours. But every time I go on a diet to lose some weight, I will wake up & automatically think about what I want for breakfast or what time of junk food I wish I could have, & my stomach will even growl sometimes!!! But this NEVER EVER happens when I am not on a diet. I don't even crave junkfood but only when I go on a diet, that's when I start wishing I cud have pizza, chips, soda, etc....Does this happen to anyone else?? Is this ALL IN MY HEAD???