Why is World of Warcraft so addicting

WOW is addicting not only because of friends, but because of the continuous story lines that are made with quests, etc. People really get into the story. The content ability to upgrade your equipment also draws people because they want to be the best they can be. The sheer size of the environment makes people want to explore it, because there are always hidden things that you may miss if you don't go beyond that last hill, etc.