I lost a lot. 133 pounds (I had so much denial about it, I hadn't even done the math right and thought it was 120 for years. Duh.). And I gained it all back plus 18. It was horrible. There were several factors that I hope I am getting under control this time.
1. I was under so much stress at the time and the only way I wanted to feel better was to eat. And eat I did. Along the way I would think, I'll stop soon, I can stop at anytime, there's no way I'll gain it all back, blah, blah, blah. All lies. Sound like any addicts you've heard of? I'm working on learning new coping skills because stress is part of life.
2. I was disappointed that my body didn't look the way I wanted it to look. I looked terrific in clothes, but if I'm being honest with myself, out of them I thought I almost looked better fatter. It was the saggy skin. This time, I plan to have it fixed. Everyone's different (and I have the utmost respect for those who are okay with it), but I would feel better about scars than being saggy.
3. I got lazy about eating and frustrated that I couldn't eat like my always-thin friends and family could. I just can't handle as many calories as they can. Period. I'm okay with that now.
4. I was unprepared for the attention and expectations that came with my new body. I really hid behind my fat. Suddenly I felt totally exposed. My outside no longer matched my inside. And while you can be happy with your new exterior, the contradiction between that and your old identity can be hard to reconcile. Part of me was upset that people who didn't want anything to do with me before now wanted to be my best friend, date me, etc. My parents, who always said they were proud of me, seemed prouder than before; showing me off. This one I'm not sure how I'm going to fix, but now I have a therapist AND 3FC so I hope this will be easier next time.
Anyway, it's a good question. I think it's true that losing the weight is the easy part - keeping it off, and not sabotaging yourself, are the hard parts.