I have experienced all of the following within the last year-ish:
1) Get into a relationship, decide you might want to marry the person, and then get broken up with. I lost a lot of weight in a short amount of time with this. It is not a very dependable method, however, as I feel like one's body can react on the other extreme, in which case you just put on the pounds, probably only making you feel that much worse. Not only that, maybe the person won't break up with you. Maybe you actually will get married, and then you'll put on more weight (although maybe you won't care then). So this isn't a very good route to go.
2) Get sick. I just lost three pounds in two days that way. Once again, I don't think this is very dependable because you can't count on being sick for appropriate amounts of time to lose weight and not have to see a doctor. And being sick isn't fun, either.
3) Diet. This is my most successful method for weight-loss so far. The drawbacks are that you have to be willing to do it FOREVER. And you might be cranky at first. However, unlike the first two methods, I think you come out of most of the cranky phase and can still have fun with life.
I am hoping to implement another method soon, which I have used before - exercise. I haven't been successful at losing weight through exercise for several years. It took working out for quite awhile each day to see results with that, and I'm not willing to put that much time into it on a regular basis. I was good at running for awhile after the last breakup, but that didn't work. I mean, I felt like I was in better shape, physically (probably mentally, too), but I didn't lose weight. Now, if I could run in conjunction with my diet, that might be a breakthrough for me.