Nick Yee conducted a project about friendship formation in MMORPG. It seems pretty interesting and well documented on this website.
While there are many factors inherent in MMORPGs that facilitate the formation of relationships online, it is also equally clear that not all players choose to do so. There are players who consciously do not bring their real lives into the game. Another way to look at the phenomenon is to ask whether certain individuals are more predisposed to these online relationships. One group of such individuals might be teenagers who are struggling with identity and acceptance issues in real life. Many teenagers feel they have no one to talk to because it is their peers and family who are part of the issue they are trying to deal with, and the online environment might present itself as a way to talk about them with someone else. But approval and acceptance are needs that everyone requires to sustain a normal level of self-esteem. Individuals struggling with self-esteem issues might therefore be more likely to form relationships online.