I thought of this earlier today as I was reading stuff on the site and wanted to get it out there.
There are a bunch of big reasons why I do not like to group up in MMOs, reasons which have actually made me quit games. First and foremost it makes me feel, as one player, weak. It makes me feel unheroic and very vulnerable. I like a challenge, don't get me wrong, but I also like to feel like I am a hero in posession of some truly unique skills, skills which can keep me alive no matter what. Many MMOs have content that you cannot solo as it's simply impossible to do so. Even non-group quests can be this tough when you're playing on your own (EQ2 had some difficult content to push through solo). LoTRO is a major culprit here I think as there are a bunch of things that become extremely difficult to do solo. I enjoy dungeons but LoTRO has dungeons you absolutely must have a full group for and it gets on my nerves.
Another big thing that ruins games for me is the constant need to group to get anything worthwhile. The best gear is almost always obtained through instance runs that require groups. This can happen over a single run or many runs depending on what you're trying to get. I'm not always in it for the gear, as I said I enjoy dungeons and would love to do more DDO style instances. It all goes back to my desire for more singleplayer quality content for solo players.
Finding groups can also be a big thing for me. It takes time to find one and once you do the fun doesn't end there. First you have to make sure the group is balanced usually the typical formation of a tank, healer, and a few damage dealers. After this you have to actually make sure your group is capable. PUGs (pick-up groups) can be a nightmare when it comes to making sure everyone can handle the task at hand. Sometimes a dungeon will require you to have already obtained certain gear to be able to weather the storm so to speak. Most of the time a PUG is lacking in skill, knowledge of the dungeon, proper gear and voice comms-things that can make or break a run.
I enjoy most games until the endgame content. Not because it's too hard (I spent 5 years in WoW and raided up until WotLK) but because it requires vast amounts of time and people to do. Raiding becomes a necessity and it gets old fast. You can only run a dungeon so many times before you realize it's the same old shit and you leave. I'm still a firm believer that a single hero could slay a dragon and as such I don't think we should have 20-40 skilled heroes needed to take the beast down.
I guess it really boils down to the risk and reward factor, and also the time spent to obtain the rewards. If it takes too long, too many people and is too dangerous it's not worth it and it gets boring.