As other poster said, we don't like to accept responsibility and admit our own mistakes, errors in judgement, slip-ups, or whatever you wish to call it.
Also, calling getting healthy a "diet" means that you will stop at some point, and go back to "other" methods.
Getting in the mood and mindset that it's a lifetime of healthy choices, sometimes helps keep a person focused and motivated.
Seeing an occasional item as a "treat", and "indulgence" or a "fail" are all methods of undermining your decisions. See everything as part of a meal plan, and plan for its inclusion (obviously, not daily). Track it, as you would any other item of food or drink. In this way, you'll also see patterns, and be able to identify the timing of what and when you can add varying items back into your menu.
That's why, when done correctly, a plan like Atkins or South Beach, that begins austere, but adds things back slowly, so as to control cravings and reactions, is effective. Unlike "diets" that eliminate certain things completely, leaving you with the problem which started this thread, a well-balanced menu can include EVERYTHING you wish to eat - in quantities that are appropriate for you.