Video Game addiction is a touchy issue. Many people see it as a downside to video games, and countless breakups have been blamed on addiction. Video game addiction isn't any different from getting really into a book or movie, with the exception of online play. Addiction really isn't the right word, but since it's commonly used in this case, I will use it.
When you're playing a single-player game, you are able to stop or pause whenever you want. Or at least, there will be a save point somewhere within the next hour. These kind of games are almost analogous to a good book or movie. You play them and keep playing them for hours on end, not because you have to be playing at that time to win, nor do you have to be playing at all, but rather because you want to. You don't want to put down the game because exciting stuff is just around the corner, or you want to see the next gory fight. Your play isn't regulated by your guild or server times, rather your play time is regulated by how much you want to put into the game. (Note: certain MMORPGs fall into this category, namely those that have little or no community involvement).
Group MMORPGs like World of Warcraft, rely on groups of players being on at once to play together. Most US guilds raid at around 7-8 PM CST, and raids last around 3-4 hours. Since there are still 4-5 end game instances, players are raiding every night to get the best gear. Most guilds run a participation system, where you get points for each raid you go to and use points to bid on gear that drops. So, in order to get the best gear, you have to be on at a certain time most nights. Since your time will be taken up in the raid, in order to make money in the game to raid, you'll have to devote even more hours to the game. This cycle repeats until you spend almost all your time in the game and loose track of the real world. Other players are largely determining when you play, and that is where the addiction lies. You become addicted to the quest for the coolest gear and obsession with your guild and raids. Because you devote the prime hours for going out and having fun every night to WOW, you loose your girlfriend, go to your job tired, and stagnate at life. There are players who can manage all of that, but it is difficult, and many more people have WOW being the dominate force in their life. I know personally someone who lost a fiancee (who was living with him, and his wife in all but the legal sense) over the breakup of their guild and the fact that the guild in game was more important than her out of game.
Now, I'm not saying games shouldn't be interesting, I'm just saying that getting too far into a game is bad. A game should be immersive, that's why they are fun, but not reliant so much on other players that you can only really get the best gear by playing at a certain time and devoting a certain number of hours to it. It's a difficult balancing act, and I've yet to see a game that does it well enough.