Never really was thin, unless you count childhood. Started gaining pretty bad when I hit puberty, and was somewhere between 250-275 throughout high school, where I've stayed a good chunk of my life.
Started to gain more with my first boyfriend (I was a late bloomer, in my early 20's). He was overweight too and we went out to eat a lot. When he got a job in another state, I moved in with him, where things between us went all to h3ll. It was a combination of many things; eating a lot of fast food and processed goodies from the store, eating as an escape, eating because I was severely depressed, having out of control arguments when I actually tried to eat healthier (I defintely knew how to eat healthier as I'd lost weight before, but he'd lash out at me if I didn't indulge with him). Talk about gaining weight fast, I put on 100 pounds in about a year, on top of already being 100 pounds too heavy!
I think part of the reason he'd throw tantrums was that he didn't have to feel guilty about what he was eating if I had the same thing with him, and of course there's the old standby that he was afraid I'd get confident enough to leave him if I was more attractive. And of course at the same time he'd attack me for gaining weight. H3ll, when I lost the first 40 pounds and started dressing nicely and wearing makeup again, he attacked me for "letting myself go" and tried to convince me that I was too fat and ugly for him to be attracted to me anymore. Ugh.
Anyway, the point was that I was in an emotionally (and sometimes physically) abusive relationship, was constantly arguing, was extremely stressed, knew my father was dying, and I was eating nothing but crap and apparently a lot of it. That part of my life is mostly just a huge blur now, and I was admittedly drunk throughout a portion of it. I'm in a much better place now, no longer drink or smoke, have lost that 100 pounds I'd gained while with him (but still need to lose that other 100!), and hope my life will never get so messed up again that it will return to that blind, out of control blur.