I can tell you exactly why i Solo/Duo in just about ever MMORPG that i've ever played. Well, i'll be honest, it won't be so easy to explain as i may have made it seem with that statement.
In all honesty... the major reason is that community has changed drastically from what it used to be when MMORPGs started to gain momentum. Even multiplayer gaming communities have changed drastically. There was a time whne playing an FPS or an RTS or any competative or team oriented game was about getting to know the people around you and challenging yourself against their abilities. It was FAR les about "proving" who is more bad ass or insulting or simply trying to win at any and all cost. I hate to use this term, but there used to be a sense of "honor" in gaming. I suppose a better word is "respect".
However, there are other reasons. I prefer to Duo because i usually do it with a close friend of mine (The only one that also plays MMORPGs). We can talk about what ever for hours on end while killing hordes of what ever, thus anything grindy simply becomes, as was mentioned, the little game played between donig something entirely different. If i play without him, i'm obviously soloing. I don't like grouping because i don't enjoy dealing with people. I don't like finding them. I don't like /NOT/ talking to them (people don't talk about anything important if at all to each other i nthese games any more). I don't like relying on them. And i don't like the way people "share" (You know.. the priest who has a warrior alt so he feels he has to /need/ roll that blue battle axe that the game prevents him from trading to his character any how? ).
Once again, i solo/duo because i know what i'm dealing with that way, and because i just don't like the way games act now adays. When ever i defeat someone i na duel or challenge, i don't brag about it, i don't tea bag them, i dont' call them... obscene things probably best not written here. I respect their efforts, i'll even help them get better, or be waiting for the next challenge. I enjoy using my own wits and abilities (less my characters pre programmed parameters) to defeat things or solve things. This is also the appeal to Solo/Duo play. Things are harder. Fighting something in a group may make it ... easy or average in difficulty. Fighting that same thing, maybe with a level or two more thna usual by yourself or with a friend, and having a chance of victory that's based entirely on you spending time learning what tactics and strategies are available to you and how you can impliment them is far more rewarding of an EXPERIENCE (not necessarilly in gear or actual character xp) than rolling through it with a group. Atleast that is how i feel about it.
Lastly, i'd like to post my "suggestion" to fix this. Scalability and cross-level missions. One of these is seen in such games as City of Heros. Basically, how many people you bring, and what level of difficulty you set, determines how much you face and how hard the fight will be. To me, a system like this should be in just about every game. Why? It means that when you DO group, it's WORTH it, and when you don't group, you are slaughtered simply for trying to paly the game you bought and continue to pay for. And if you really want to go "Hard core" you can at any time you desire. When done right, it can be great for Solo, great for (my favorite) SMALL groups, great for full groups, and great for guilds all in one simple mechanic. Once again, Scalability. As for my second suggestion, cross-level missions, it is based on the concept of there being a pool of missions based on the faction or what ever that you are on, that is tailored to different levels, but still is inclusive of all of them. As an example:
Let's say level cap is 50. A couple of level 50 "generals" are in a guild and they have people of the "Commander" and "Leutinant" range (40 and 30 respectively). These sub leaders have people of various levels below them, from Recruit to i don't know... Veteran. This scenario is obviously a large scale one, something like defending their territory from a horde. Basically, the quest is generic "Defend your Castle", however the PARAMETERS are different based on your level. The parameters can be as simple as Recruit needs to kill 10 ... frog men and Leuitinant needs to kill 100 frog men. However, it can easily be something like Recruit/Veteran need to kill X amount of the Fodder/Soldiers while Lts need to take check points, Commanders need to take down base cames, and Generals pretty much need to win the battle whether that is defeating a boss or leading the charge to the main encampment. How do you keep the lower levels entertained when it gets to the end? Have the last fight basically be them holding off an "endless" swarm so that the mid levels can take down the help the boss has while the higher levels take on the boss itself. And of course, with Scalability also in play, even a clan full of "generals" can have a challenge as the quest would adnjust to what they are able to forseeably handle. This scenario can be palyed many, many different ways.. throughout a dungeon, in an open field... ect. It doesn't matter where... what matters is that no one is useless simply because they aren't the same level. Everyone can play, be part of something, and help the team succeed.