One of the main things that keeps us playing MMOs are the communities that populate them.
While some players can, and do, play their chosen game solo, for most it's the social component of these titles that pulls us away from those stunning graphics of consoles games and RPG titles for the PC. By comparison, while they're beautiful, many of those games seem 'empty' after having spent time with a massively multiplayer title.
Communities in whatever form they take -- be it a guild, alliance, or corporation -- are essential to the success of an MMO, but interest in playing a given title can wane over time. Likewise with the social aspects of these titles, tempers can flare and patience wears thin. Sometimes you find you just don't have that much in common with the people you're playing with and it's time to move on.
Bearing that in mind, are the people you choose to play with in your MMO of choice the same people you started out with? Do you see any parallels between how the company you keep in-game changes over time and how your real-life relationships with friends is always changing?