One of the hard parts about losing weight is that you are "starving" yourself just a little everyday if you are burning more calories than you are eating. There is mental fatigue associated with this, but there is also a physical response. Your body doesn't realize you want to be small for arbitrary reasons. It has other priorities.
People sometimes talk about the "starvation response" where your body plateaus on weight loss despite your following the same plan of exercise and diet. Some people doubt that this actually occurs, but it does make sense from a biological viewpoint. Your body values stored energy, and is alarmed by the continuous loss of that energy. It is in the best interest of your survival under adverse circumstances for your body to adjust your metabolic rate and stop burning calories at a high rate if its system perceive that you are consistently putting less energy into it.
The fact that the body responds badly to long-term reduced energy consumption is also reflected in the fact that your cells are damaged in their ability to deal with water when you diet for a long time. Water retention (and subsequent "whoosh" responses when one looses several pounds seemingly overnight) is related to this type of damage. When you undereat for a long time, the cells can't let the water out because of the damage they sustain. If you eat more, they return to their capacity to allow water to escape and you lose that water weight.
I don't know if there is absolute scientific evidence about starvation responses, but I do know what my body feels like and how it responds to long-term calorie reduction. Every once in awhile, I have to eat for real. That is, about once every two or three weeks, I have a strong physiological need to eat between 2000-2500 calories. The hunger I feel at those times is very base. It strikes at the very core of my body and my human existence and is nothing like the daily hunger pangs that come physically or emotionally as I diet. It's my body shouting, "I'm starving."
When I have these experiences, it is an order of magnitude harder to resist those urges than the normal daily urges. It's much harder to simply dismiss it and go to bed hungry or to say it's emotional or psychological hunger. When I get like that, I want meat, eggs, cheese, and nutrient dense foods. I'm not craving sugar, carbs, or enjoyable little treats.
On those occasions, I eat. In fact, I eat until I feel full rather than simply satisfied. I realize that these days could be an issue in my long-term progress if they happen too often, but they don't. To me, the most important point is that I can tell the difference between this strong biological response to prolonged calorie deprivation and other types of hunger (physiological or psychological). I don't feel guilty for "giving in" at those times, because I think my body knows what it needs and it's best sometimes to go with the strong signals that it gives me.
For the record, these periods of what I feel are a strong response to long-term "starving" are becoming less frequent, and I never overeat massively when this happens. I think it's actually better in the long run to act on these responses because I do believe in the starvation response, and I think effective long-term weight loss isn't as simple as continuously reducing calories and burning more energy without break or variation.