Why can't you have just a little piece, it won't kill you?!

Let me answer that.  BECAUSE once you allow yourself a bite here and maybe a small piece there, it is very hard to stop that flow and it becomes like a snowball rolling downhill.  It is not the one small bite that will hurt you, it is the change in your mindset that WILL.  Once you allow yourself a "treat" it becomes harder and harder to tell yourself to stop and that this will be the last one.  Therefore, abstinence is the only way!  :)
Oh of course I say this only about myself as I can not speak for others, but it is prime example of what happened to me Christmas Eve, Day, and the day after.  Now it sucks getting back on track, but that is the only option.