I've always wondered if it was some sort of grammatical rule or if it's a way to disassociate the excess poundage from the obese or overweight person...like, a way of saying: "That fat and blubber isn't PART of me which is why I'm trying to get rid of it!" "Losing THE weight" just falls oddly on the ears too...I'd think the person trying to lose weight would want to OWN it! "It's MINE, I'm losing weight, MY WEIGHT!" more than anything else. Anyone know? Is the definite article "the" some sort of Jenny Craig psychogical tactic for these people?

Plain old "lose weight" sounds slower and in less quanitity. It's indefinite, kind of loose and unstructured. Losing "the weight" sounds like getting rid of all of that excess flub you want to get rid of, not just some weight. People want things fast and want to be reassured, not floating around with pounds. They want it all gone.

Referring to weight as "the weight" means its an unwanted thing that is not considered part of you, its like a parasite you want to free yourself of. People then associate THE weight as some sort of enemy that is hiding the real person inside.

It's to make it sound disgusting and emphasize it as something external that shouldn't be there. At least that's how I see it. Usually when you say something is yours, it's something you're not ashamed of or whatever. I think it's more of a psychological wordplay thing than an actual grammar rule. But I'm speaking out of my *** so it may not be true.

Think of it as short for
losing the EXCESS weight. That suggests there is an amount that is too little, and an end goal.