It is essential for the body to have a bit of all foods - carbs, iron, protein etc. Diets don't work as they cut out essentials.

Breakfast is essential as long as it is reasonably healthy - that gives us the proper body's sugars etc for the morning until lunch.

Weight loss too fast, drains the muscle from the heart and often results in heart attacks or worse. Then it hits the body's organs - slowly (if annorexic it's quicker), but still affects nonetheless.

Each person is built differently - different muscle ratio to the next person etc, so it takes less time for one person to ruin themsleves than another person.

People say "Oh, I only do it a bit and only cut out this that and the other...." fact is, we cannot cut anything out, just cut down, take extra walks, a bit of cardio vascular excercise too and that's all it needs really. 

Diets are to make money for those introducing them with their books etc, that's all. There is a very high awareness out there, with those doing those diet books, that people are weight conscious, so that's what they prey on.