I got addicted to an MMO (DAoC) long ago. Luckily for me it didn't cost me much money other than the bot accounts I bought and maintained. But, it did cost me other things. I dropped out of grad school, stopped working out, stagnated at my job, and became completely obsessed with the game. I treated it as a second job. Looking back the why seems silly and embarrassing - I was part of a guild that needed my help. Eventually I had a long term relationship end over the game. The next day I gave all my accounts away and never played again. I still play single player games when I have time, but definitely no MMOs.