Because when you eat "nice" food your body releases endorphines into your blood stream to say thank you for feeding it. Basically your hooked on your bodies own pleasure drug.
No food is inherently "nice", you find some nice and obviously those are the ones which you find hardest to refuse.
You could say, "why is it easy to reject horrid food ?", easy 'cos it's horrid !
Mr Poohead, you are the enlighted one. I have also read your question about weird sleep and although I have no answers it was most entertaining thing I've read.
I have very vivid dreams and am sometimes very unclear about whether they are real or not. The night before last I dreamt of a crawly in my bed. I dismissed it as dreaming and woke this morning to find a fat spider floating in last night's cup of tea. He had obviosuly returned (cocky fella) but met his end. Sleep is a weird and wonderful thing indeed.