I may join a PVE focused guild when my Mesmer reaches her mid 30s and is ready to do instances. For now however, I'm going solo, representing our family guild of alts. My Guardian is still in the monster sized guild she started in, which for me, is a bit too impersonal. It's a nice guild with helpful people but my personal tastes lean toward smaller, more intimate social settings. Consequently, I'm often doing my own thing on her too.
The lovely thing about GW2 is that it doesn't matter if I'm in a guild or not. Guild membership does have its perks, depending on how your guild spends its influence. However, they're not very necessary in the early part of the game. And even when I'm flying solo in GW2, I never feel alone.
Game mechanics have encourage players to behave more like a community out in the world. We don't have to fight over harvesting nodes or loot. Everyone can resurrect fallen players. The more activity there is in a zone the more likely events will escalate to the next level. Down ranking means high level characters can't run through zones, one-shotting mobs low level players may need or razing the harvestable resources. This eliminates creating annoyance and animosity across player levels. Combined, these elements foster better behavior and a feeling of citizenship.
On this character, I'm hanging out longer after events to see what I may have missed the first time through. For times when a group is needed to reach a POI or skill point, it's been easy enough to ask in map chat if anyone else needs the same objetive. People show up to help then disperse back to their own little worlds. Yeah, GW2's mechanics really really suit my playing style.
Warhammer started the demise of needing to physically form groups. Rift took the mechanic even further. I like to say that GW2 not only eliminated the need for grouping outside of dungeons, it also reduced the need to communicate with other players in any form which is fine by me. Now chat is casual and more banter-like versus players spewing informational walls of text. The game makes most things pretty obvious from a mechanics point of view.
I continue to solo, has I have from the beginning of my MMO journey. It's me - my thing. The biggest difference is the game mechanics don't punish me for being solo or reward people explicitly who prefer grouping. In GW2, we cohabitate harmonuously and on equal footing.