I am a soloist, and I enjoy the dungeons in FFXIV.

I play solo because most people online are rude, childish, arrogant, and often know how you should play your character (because if you are not doing it how they do, you are obviously wrong). Its not worth wading through the muck to play with those good players. I have yet to find a FC/Guild in any MMO that didnt have players that made me feel unwelcome, so I just solo so I can enjoy the game, because its not enjoyable otherwise. Its not that we are anti-social (although I am sure there are some of these as well) its that the internet offers an anonymity that breeds people I would rather not socialize with.

Why do I play an MMO then? Well an MMO is huge, constantly updated, new (official) content, features, and game play elements for many years after launch. No offline RPG offers this (and user generated content or mods are not a suitable replacement).