To lose weight. The fact is that most people will gain weight when training for their first marathon unless they watch what they eat. This is true. You're going to be hungrier, and it's easy to adopt the mindset that you can't possibly overeat when you're doing all those miles. Well, guess again; it's quite possible. On the other hand, if you're sensible and healthy about your diet, then you will lose if you want to lose.
Because someone else wants you to do it. It's easy to get sucked into someone else's enthusiasm, and often that's a fine thing. All of us have committed to races just because our friends are doing them, and we don't want to miss the adventure; however, a marathon is a whole different animal. To do it safely, you must train purposefully and religiously. It requires a big commitment that will affect all the other people in your life. You need to want to do it for yourself. If you do, great, but if not, you will not have the drive to stick to the training and prepare properly.