one reason is, you are not as in control of your weight as you think you are. The average person gains about 250g a year. My lecturer worked out that for this to happen, you would be eating one extra chip a day worth or excess calories. The point is, our brains try to prevent change in weight, and they are very good at it (hence why losing weight can be so difficult). So if you lose heaps of weight really quickly, your brain doesnt adapt, it thinks something really bad is going on, and tries extra hard to gain it all back. Pretty much, we think we're so evolved but really, we still have a caveman hunter-gatherer mentality when it comes to food, where we subconsiously think we may not eat again for months, so weight lose in that sense in bad.
Yeah. Often people use very unreasonable methods of dieting when they go to lose weight, such as restricting food intake too much or working out too much. When you go back to eating normally or at a reasonable level or don't work out as much, your body gains everything right back. So it's recommended that you do it slowly, properly, and safely. This way you're treating your body much better, and you'll have better results in the long run.
If you don't do your best to stay in shape, you *will* gain that weight again. They want you to be on the right path when trying to stay healthy, you know you don't want to become obese and soon have heart problems and even rely on others to live your own life.
They're pretty much trying to say, do your best to live a healthy life and once you're on that road, stay on it!
Usually when a person looses weight to quickly it was done in an unhealthy mannor. When you loose weight the wron way you usually gain it right back because those weight loss tricks you used to loose the weight wont last - unless you have an ating disorder. Losing weight and keeping it off takes work and when you cheat...it always comes backk.