I agree it's most likely down to attitude and sustainable lifestyle (diet and exercise), at least this seems more likely to me than there being a certain speed of weight loss where you voodoo your metabolism so it never works properly again. Lots of people lose weight initially on extreme but unsustainable diets and then gain it back; how are the Biggest Loser contestants different from everyone else who's gone through this, other than notoriety?

I think people should eat less than they burn if they want to lose weight. You can even have iced cream, just smaller portions, and you might have to work some of it of. Deny yourself iced cream, if you have a sweet tooth, and you're more likely to say "screw it" one day. I'm sure this stuff is correlated with how quickly you lose weight because bigger deficits would make it harder and that's going to be less easy to sustain. But I'm almost sure it's the person's ability to stick to their calories and not that having lost weight quickly in the past broke them and doomed them to a life of obesity forever after.