Massively multiplayer online role-playing games historically had a very important social function. In the early days of the internet, and up to the times of World of Warcraft they served not just as an entertainment medium, but as a platform for various communities as well. I, however, found my people via regular chatrooms and bulletin boards. And so MMORPG presented an interest to me only as games, and solo-able games at that.
I played Anarchy Online, Ragnarok Online, EverQuest 2, and WoW, but only interacted with guilds and other such systems as a necessity, because frankly you need a guild to have a good time long-term. The solo experience, even when possible, boils down to basic mechanics and completely locks you out of late-game content, while communal events, like seasonal quests, don't really bring players together on missions. Instead, you just retread each other's steps and hang around same vendors for a bit, and then its over.
On paper, the first Guild Wars game wasn't even a classic MMO. It's much easier to compare it to modern live multiplayer games with hubs and limited scope instances to explore and quest in. And I enjoyed it well enough because of that. Then the sequel came out, and that was the freshest look at MMO classic games I have ever experienced at the time!

Guild Wars 2 doesn't have typical quests. There are no people with a yellow exclamation mark above their head, promising you rewards for fetching N of X or killing M of Y. Instead, you have story missions, scripted and instanced encounters within world levels, akin to the levels from the first game, and you have a waste open world that always lives through one problem or another.
And you can join that living world at any point. Each area of the world map has unique events going on on a repeating basis. These events are often chained together and show, with fully voiced NPCs a certain plot unfold pertaining to the area's state within the larger story. You are notified about these events, they are very much visible as areas on the local map, and they very effectively make players flock towards them. There is always a reward, which gets better with more involved participation. Events can fail to, and have level of difficulty about them. Some events can be triggered by players, and these can be more difficult than others, but offering a helping hand is practically free, and why wouldn't you slay another mob alongside another adventurer?
This loop creates an incredible feeling of being a part of a massive game without forcing anyone to join up in a guild or a clan. Instead, cooperation comes organically. To aid with that there is even an ad-hoc party system where one player starts a farming run and invites whoever is interested in joining, and like a whirlwind you run through the paces together, sharing the difficulty and earning good good loot.

Once you get far enough along, world bosses and some expansion maps become places where spontaneous cooperation really shines. Some players become commanders even, and rule over the battle plan to overcome specific encounter mechanics. Though other times it's just a fun brawl, especially when a really significant amount of players joins the event.
Outside of public events, which I adore so much, and more classic dungeons, which I'd enjoy more if they were viable in solo play, there is another big aspect of Guild Wars 2 that gets me hooked. Map completion. Each area has a lot of things to do for you personally, without direct help from other players. There is still a bit of "normal" questing in the game's DNA, there aren't exclamation marks, but there are hearts. Hearts mark are local NPCs which have an ask of you. In an area around them, you need to do any of the provided activities, filling their progress bar until completion. You can do that at any time, leave and come back, and, if areas overlap, even combine with world events. The tasks are regular, and the system is light, but its purpose is to lead you in and get you accustomed.
While you're venturing and adventuring, you will also run into vistas, high and obscure points on the map providing a certain traversal challenge. There is a lot of platforming and parkour in this MMO game! Some vistas are only there as a reward themselves, giving you a bit of appreciation for the world and the lore, and some lead you into more areas with more things to do.

But while vistas are light in platforming, there are far more entertaining challenges too. Hidden jumping puzzles, as the name suggests, are all about testing your ability to navigate and climb and, of course, jump. And they are hidden, such that you can find them by just being curious and exploring nooks and crannies of the world. These encounters, and other secrets, can be pretty involved, require you to pay attention, do specific things, test your knowledge and quick thinking, or precision and creativity when problem-solving. They are always fun!
And you always get something for your efforts. Besides direct rewards and reward shops associated with events and encounters, there is also an extensive system of achievements, which provides yet another layer of questing and engagement. Achievements vary from basic tracking of your actions to hidden quests that send you exploring certain parts of the world, to end-game meta activities, like legendary crafting.
I have returned to Guild Wars 2 recently after over a decade of absence. I found my character preserved, with a ton of various rewards from years I had missed. I finished parts of the base game that I never got to before, explored the entire world of the game, completed every heart and found every vista, and it was glorious. And I bought some expansions and then set the game aside again. So I have something to return to.