Back when I was a raiding guild officer, I was around Azeroth just about any moment I wasn't busy elsewhere. There were applicants to interview, recruits to orient, newer players to help gear up and oh so many other to-dos to check off.
Now that I'm in another of my PvP periods, my attention span is less tied to external motivators like guild tasks. I recently leveled a new character to the precise bracket I'd decided I wanted to play him -- and then never logged him back in. Instead, I hopped to another realm with the idea of quickly level-capping a character who was just shy of the finish line. That project quickly devolved into accidentally-on-purpose choosing quest areas that turned into world PvP skirmishes lasting the entire evening. I'll get level 89 Hordeside some day ... Some day.
With such scattered motivations, my logins sputter and spark, hot for a week and cold for two, on a run for a month solid and then spare for the next. The single thing that's probably most responsible for provoking a good, long spate of WoW time is watching my daughter play. As soon as I hear the clarion call of a turned flag in Arathi Basin, I find myself asking her how much longer she plans to play so that I can use the computer. Once I'm in, I have to visit all my girls, and then the new projects start piling up anew ...
What single thing is most responsible for keeping you logging in regularly? Would you be long gone if not for your WoW buddies? Do guild responsibilities keep you happily busy? Tell us what keeps you coming back for more WoW.