Not to get all psychological on anyone, but I struggle with this too (I've been up and down between 205-220 for 6 mos.) so I looked into it. I've read that 15 pounds is a common time for people to give up because that's when the weight loss starts to become apparent, that's when we notice change. A lot of people struggle with change in general, and often subconciously. For people who have been overweight their entire lives there may be apprehension hiding behind all the excitement about getting thin. After all, you look different to yourself, others, and image is a huge part of our society and the way we view the world. Things like having to live up to your new figure (sometimes people associate being thin with being their ultimate selves and dont feel ready). Even simple things like worrying your friends will think you're vain for focusing on dieting, something that has some stigma among women who focus the majority of their lives on pursuits outside themselves and want to viewed by the world as intelligent. Of course, there are flipsides to all of these -- my point is that weight loss is about more than calories in and calories out, it's emotional too.
I haven't been overweight my entire life, actually it's only been two years. But there were some difficult things going on in my life when I gained the weight, and finding out about this study and change made me sit down and think about how I actually felt about losing weight. I realized that everyday I think i want to lose the weight so much, and I do, but that there were some related things that definitely worried me about being thin again too.
Just some (food) for thought, haha. I'm no expert, but I couldn't help but pipe up on a couple things that hit home on this board this afternoon - and this is definitely one of them. I'm working on this, and trying to avoid self sabotage daily. Nice to know I'm not alone on this rebound thing.