I think women are judgmental. While your woman usually sticks with the whole you, she is still always comparing individual features to others out there, and she believes that she is “settling” for you. So even though you have the best personality she can think of, and you are good to her and loyal to her, and you are good in bed, she is still comparing your dick or your body to some other guy’s out there and thinking “there is another guy that has everything he has PLUS a bigger dick and a better body.” She is of course wrong - nobody has it all, but she still takes your other great qualities for granted, and it leaves you with a fear that she will be disloyal to her because of some other tempting fruit. I guess we call that “insecurity” on our parts.
I think that men are not as selfish as society makes us out to be, and we do want to be the best possible human being in every way for our women. We don’t want to be the one she is “settling” for. However, whereas things like loyalty, intelligence, bedroom skills, athleticism, and even money are things that we feel we can control and alter in some way to improve ourselves for our woman, dick size has always sort of been something we feel we are stuck with. I think this site makes us feel like we can have control over one more aspect of ourselves, and that feeling of control is addictive.