Hardcore gamers want the adventure and fame that their life doesn't have. If that's an addiction, we are all addicted to this same thing. Everybody wants more: Some buy lottery tickets, some watch exotic TV shows or read books, games are no different; they just allow you to fool yourself into believing you are accomplishing something.
So what does that have to do with why I don't MMO you ask? Well, it's not addiction; I'm addicted already, there's no fear of that. I enjoy the hardcore and the casual but what I don't enjoy is continually paying for it (I'm a recovering pirate for the sake of Piper's Pit, I'm used to paying nothing and you want a monthly fee?) So you buy the game, you pay for your internet, you own... Thirty days of free play before you have to cough up more to keep playing. Wait - isn't that called a demo? An Open Beta? You can't play what you've bought without first buying it again, and you're ok with this? What the hell did you buy to begin with? If you bought a car, straight up, and the dealer told you that part of the agreement was to pay 1/3 of the cost every month to continue to drive it, what would you tell them?
Worst of all, the more you pour into your character, the more attached you become to this imaginary object. Why do people love the Portal Companion Cube so much? They spent nary a level carrying it around and found it hard to let go at the end of ten minutes. Imaging those who spend months, hell years with their character? Sure you can stop playing anytime, but you can't back up your character. The best you can do is continually pay to keep them around. For all your efforts, in the end is an erasure of your existence and a blank character page. Any wonder why it's so popular? Nothing evokes the aspiration for life quite like Death.
So what do I tell every MMO player I know when they try to lull me in? The same thing I would tell that fictional car dealer, "I don't buy something for the privilege of renting it every month".