Personally, I don't find these forums all that addicting. Mainly I monitor them to keep my finger on the pulse of the industry where I make my living. There are times I post more and make a point of checking for responses to what I post but, even at my most prolific, there are lots of people 20+ times as active as me.
The original question is an interesting one to me. Based on my education and experience, I would have to be considered something of an expert in addictions and behavioral matters. Apart from the tons of posters I see who are transparently gambling addicts in denial, I have observed many people who clearly are addicted to these forums. However I have learned there's absolutely no point me giving an honest, comprehensive analysis of that circumstance. Not here. I have broached the subject at times in threads such as this - meaning to be helpful - but I have found that really getting into the nitty gritty of this topic in the forums themselves just makes me an *******. There's a lot of negative stuff that comes out - it's unavoidable. A lot of people don't want to hear it.
Anyway, I doubt the intent of the question was to actually talk about literal addicts. I can't help but take it that way due to my background but I think the intent of the question was, for lack of a better word, nicer. So I'll keep things warm and fuzzy and say the forums are so addicting because of the friendships that develop.