Why It's Depressing
We'll tell you who wouldn't be motivated -- anyone who has met Ryan Benson in real life. Benson's current weight is around 300 pounds, which is just 30 pounds less than what he weighed at the start of The Biggest Loser.
Benson isn't an anomaly -- almost every Biggest Loser winner has gained back a chunk of the weight he or she lost on the show. The worst example is Season 3 winner Eric Chopin. Chopin began the show clocking in at 400 pounds, and won after successfully dropping 200. Once the show was over, however, Chopin bounced (ahem) right back up to 370 pounds like he got stung by Earth's mightiest bee. It's like some kind of mummy curse the contestants can't escape.
The unfortunate truth is that people on The Biggest Loser don't do anything but train for the entirety of the season -- the show's producers cover all their expenses during filming. It's not like they're going to work and then driving over to the gym to film some sit-ups. They aren't doing anything except training, under constant supervision, for however many weeks production lasts.