I think that the issue isn't "why" so much as "whether." I've seen many, many reasons for people wanting to lose in the years I've been coming here. They range from The Wedding to serious health conditions and everything in between.
The "whether" part has to do with whether a person is willing to get serious about it. Maybe you want to avoid diabetes... maybe you just want to wear a bikini and feel good about your hotness... All reasons are good reasons, if they actually lead to losing in a healthy way, to reach a healthy weight!
By "get serious" I don't mean rigid, military-style hup-2-3-4 dieting, although for some folks that does work best in the beginning. I mean, serious enough to pay attention and make conscious choices about food. Follow WW, SBD, Atkins, count calories, or follow intuitive eating--but have a commitment to being mindful about whatever it is.
My own reasons were primarily for health. Bikini? Not important. Looking good? Yes, I like to wear smaller sizes and feel that I look nice. Main concern is being healthy as I get older--avoiding diabetes, high blood pressure, cardiovascular disease, increasing joint problems. Becoming fit so that I can walk, hike, participate in activites like volleyball, swim, etc. And, having lost 50 pounds and made regular exercise a part of my life, my blood glucose is great, my blood pressure is super, I went hiking in the Rockies on my vacation in June, and I look just fine in my new, smaller clothes.
Cheers!
Jay