The social interaction, and more importantly, feeling of responsibility is a big factor for a lot of people. When I ran a dynamis linkshell, I felt an obligation (the shell could run just fine if I wasn't there, nor was anyone forcing me to go other than myself) to show at every run and would put a lot of effort into making every run (not hanging out, ditching girlfriend, letting schoolwork pile up, etc..)
A lot of people get wrapped up in the "for the guild" mentality in MMO's and sacrifice a lot for the good of the guild. Whether that means raiding 40hours a week in WoW, or camping HNM's 9 hours a day (no matter which hours...) you get a strong feeling to be there even if there's no reward for you personally.
Also many people have a drive to "finish" games, I personally try to beat 100% of any game I play. collecting whatever hundreds of small items you need for 100% in a game isn't too bad, even grinding lvlcap in RPG's isn't too bad, but in an MMO you'll never reach the end which causes a trap (these will be people who tend to have rank 10 in every nation, done all quests, have crafts up, and etc..).
The feeling of being stronger is another important factor, getting the next piece of gear can take a lot of time, and can be for a minor improvement. But wanting to be the best drives you to do things like spend weeks and weeks making gil to afford that 15million gil item that gives you +1 to a stat over your current gear. Tied into this is being competitive, I loved blm burns because I liked doing more damage than 99% of the other people I played with and I always wanted better gear to make sure that stayed true.
As raesvelg said the lotto feel is another thing, you don't want to skip out on that one run because what if -that- was the time you were gonna see your item drop, or the time you were going to win the roll for it. You show up alot for the minuscule chance that it's your turn to win something.
Someone who has trouble socializing in the "real world" may find the internet an easy place to have friends and be appreciated and loved, many people post on forums, and such for this feeling. It's much more readily available in an MMO since people will like you just for helping out, so MMO's can fill emotional needs if a person has trouble obtaining it form outside.
There's many reasons why a MMO can get addicting, and the more reasons apply to a single person, the more screwed they are. I'm personally glad I've kicked the habit with FFXI and though I played wow a ton when I did, I never let it get in the way of my social life. I'm playing it safe with MMO's from now on because yes, they can get too addicting.
Now if only I could kick the habit of procrastination... it's 4am and I have a paper to finish writing and I'm visiting forums I haven't looked at in months =P
