In a comment on Brian Green's blog, Michael Chui made a great point:
"Notice that, in EVE, I have never heard someone annoyed at the necessity of joining a corporation. It has a lot of "alone together"; my friends and I gang up, but we generally don't do missions together."
In MMOs, encouraging players to group together is just one way of encouraging them to feel like part of a community and act as a member of that community. Soloing often does not mean being anti-social. It means fulfilling your communal role while apart from others.
For most of us, when we go to work each day, there's usually at least a little time during which our tasks do not require the direct aid of others; we might even be more efficient in those tasks without others. But we are still working toward a communal goal, even as that goal coincides with personal goals.
Group-play is definitely important. Sharing experiences can heighten those experiences and ingrain them in memory. But there's a difference between encouragement and strong-arming.
Generally speaking, developers should encourage players to group without forcing them to group. I certainly agree that some goals should require group action, but those goals should generally be directed toward benefiting a community more than benefiting individual players. When dozens of players unite to slay the dragon or destroy the Death Star, it should be for the purpose of promoting community interests (possibly including NPCs in that community), rather than securing loot and xp for individual players.
MMO developers should strive to get players to focus on things beside just themselves. With that accomplished, players can then choose when to adventure with the squad or clan and when to adventure "alone together".
In real life also, friendships often begin by necessity or forced togetherness. But they also commonly begin in other ways, such as speaking with your neighbors (people near you) as you're working or playing separately.
I remember, in EQ2, I made friends with other crafters because we'd gravitate toward familiar crafting stations and those stations were near one another. We didn't work together -- we didn't even trade goods or advice most of the time -- we simply conversed about whatever came to mind as we tackled our individual tasks.
Another way people commonly become friends is through admiration or curiosity. If your crafts/inventions greatly impress me, I might try to meet with you to express my admiration and discuss the craft. If you accomplished a feat beyond those of average folks, I might want to find out how you did it. Or I might just be curious where you found a particular item or got a particular title... even if the game was too dynamic for me to expect to be able to acquire the same item or title. In real life, there's no chance of my being able to acquire a Porsche 911 Turbo or a high-end Martin guitar, but that doesn't stop me from approaching owners of those items and carrying on a genuinely friendly chat about them.
And then, of course, there's non-professional networking. I happened to be friends with your friend, so we meet up and become friends ourselves. Note that, in real life, such meetings are often not based on goals of accomplishment. Perhaps I'm meeting you for lunch, and you bring your friend with you because you want us to meet or because the two of you happened to be together at the time. In this case, the three of us are doing something together, but we are not working toward a goal other than to simply relax and talk.
Likewise, in a game, social networking may arise without any quest, loot or other reward involved. For example: if a game actually made visiting another player's character-house worthwhile, friendships might be made at gatherings which are purely social.
In short, grouping needn't be the backbone of a successful MMO.