I was 22 and had gained 10 lbs during my first year of marriage. I just wanted to lose the weight I had gained, but I lost the weight and kept going. The things I think contributed to it are:
1) Success -- I'd always been on the edge of overweight all of my life but had never really been able to get thin, even though I had tried many times. For some reason, I figured it all out and I got thinner than I had ever been before. As I got thinner, I got a lot of positive feedback and became addicted to the comments. For the first time in my life, people were calling me "tiny."
2) I was in graduate school and working as a graduate assistant to pay for my tuition, so, of course, there was stress...
3) I'd only been married a year, and I just couldn't seem to make my husband happy with anything I did. Also, he was openly repulsed by overweight women -- made lots of comments about them all the time. Losing weight and getting thin was the only thing I did that seemed to make him happy. Years later, he would come to hate that I had an eating disorder, and now he shames women who have them. Go figure. He hates me now for screwing up his life. I hate myself for screwing up mine. We're still married, by the way -- it's complicated.
I'll be 50 in September, but my ED is still taking its toll on my life, even though my weight isn't all that serious right now (107 lbs at 5'5"). For example, yesterday, my husband and 14-year-old son left for 5 days in New York City, but I couldn't go, because I wouldn't have been able to bring all the food I would need to pack for 5 days.

I always have to take my "special" food with me wherever I go, but we were going by train, and I was only allowed 2 carry-ons, and I knew I couldn't pack all my clothes, toiletries, and food for 5 days into just 2 small bags. A normal person could have managed, but I also have body dysmorphia which basically means that, in addition to my food, I have to pack tons of clothes and other things in order to just make myself look acceptable to myself every day. So, while my family is off having a good time in NYC, I'm here at home by myself -- again, and my husband is mad because he's losing money on my train ticket and tour package. I've never traveled by train before, and if my husband had just told me when he was making the plans that I was only allowed two carry-on bags, I would have told him right away that I couldn't do it, but he made the plans, and I didn't know about the luggage restriction until just a few days ago. I have no idea what he was thinking -- he's been traveling with me for years and knows that I have to take tons of stuff with me wherever I go. You'd think he'd know better by now.
