Fruit juices (the worst and most common culprit), excessive salad dressings, ketchup, and full fat (or even semi-skimmed) milk in tea/coffee are all to be avoided by the prudent dieter seeking healthy and sustainable fat loss.

Hidden Calories

It reflects our desire for instant gratification and the all too human reluctance to be patient.
Drastically reduced calorific intake shunts the body into 'starvation mode'. This effectively slows down metabolism and causes you to convert almost everything that you consume into energy-storing fat cells in case you encounter “famine” conditions again, and your body needs something to feed off.

Crash Dieting

Behind every successful person is a positive and reinforcing social structure.Surround yourself with positive, reinforcing characters who want you to succeed and buy into your long-term happiness and fulfillment.

Poor Support Structure

The majority of the calories you burn each day are determined by your resting metabolic rate (RMR).RMR is largely a function of how much muscle you have on your frame, and how hard that muscle is made to work. Therefore adding activities that promote or maintain muscle mass will make your body work harder and elevate your metabolism. Weight training is the single best form of exercise for achieving these goals.

Ignoring Exercise

If you are 50lbs overweight and give yourself five weeks to get ripped abs or tight buns, you are quite simply setting yourself up for failure.This body is achievable, it isn't going to happen overnight, but if you set yourself a goal and motivate yourself it is achievable.

Unrealistic Expectations

It takes discipline and strength of character to effect a fundamental change in any of our ingrained habits, but once the momentum has been created, and the sense of satisfaction and well-being becomes your constant companion, you can do it.

Lack of Willpower

Vague goals can scupper your motivation and progress, so too can the lack of clear and precise goals.
Draw up a plan of where you want to go, and how you are going to get there step by step.

Lack of Clear Goals

Just as a lack of goals will derail you, being overly focused on goals to the detriment of everything around you will have the same effect.
A setback (a missed goal on the weighing scales, a lapse in our healthy eating plan) is just a temporary, fleeting thing, and that we have tomorrow to get back on track.

Too Goal Oriented

Anything that is short-term and unsustainable, in other words, anything that doesn’t instil new and long-lasting habits in your food intake, is both a fad and counterproductive to your long-term success.If you can’t incorporate and apply what you do (in this case your diet) into the real world around you, then surely it is of no use at all.

Fad Dieting

Many people lack even the basic ability to distinguish between a carbohydrate and a protein. Read some books, or consult a professional – better yet, do both.

Flawed Understanding of Basic Nutrition

You already know that the key to success isn't to "diet" at all, rather it's to change your bad habits into good ones, so let's take a quick look at why it's so hard to stick to the plan. I actually think that this subject could provide enough thoughts and information to justify a book in its own right.