Now, I'm not a gainer so all I can offer is my opinions and observations, but I felt like weighing in, if you don't mind.
First of all health isn't the be-all and end-all of existence. There are plenty of people out there who smoke, drink, and otherwise introduce into their body substances that will do you in far faster than a BMI over thirty could all in the name of feeling good. I like to think that a lot of people in the community have realized that most of these health recommendations are based on statistical analysis to save insurance companies money and do not apply in any but the most general sense. It's also possible that some people don't value health the same way you do and are willing to sacrifice a little for what makes them happy. Lastly there's a possibility that people believe they won't be the ones who become statistics. Very few things people do are so dangerous that all of he doers experience the full consequences. After the first or second time you try something and it doesn't kill you it's easy to develop a feeling of being, "special" or, "invulnerable". That's human nature.
More importantly, Bearsy, you have the experience already. The grass is always greener, after all. Being fat feels good so being fatter should feel better, right? Most of the gainers out there can only fantasize and inquire about what it would be like to be your size or larger. Some idle speculation and some conflicting reports (and boy can they conflict) shouldn't discourage anyone from anything. This is a journey of discovery, after all. The things you might take for granted might be part of the fascination for others and the things you find most unpleasant they might take for granted - or, at least, learn to deal with.
Something else you might want to consider is the gaining process itself. While there are no shortage of people out there who say, given the chance, they'd gladly wave a magic wand and weigh a thousand pounds, that isn't everybody. The people I'm talking about (and I hope the majority, so I stand a good chance of eventually hook up with one) are in it as much for the process as the end result. The feeder/feedee relationship can be a fun , nurturing, and long-term one. I know that's what I fantasized about as an overly-romantic youth.