The problem is even worse if you're in the open world, questing alone. If you see someone fighting a monster, then according to the game they "own" that monster. There is no incentive for you to jump in and offer help, because you're spending your time and risking your neck for nothing. You won't get any loot or XP. You might do so out of a sense of altruism, but the game mechanics themselves are telling you "helping people is worthless". If you see a gathering node in the world where you can collect plants or mine ore, you have to rush to it before the other players or they'll take it from you. The game is telling you "other people are your rivals".
Don't help. Don't work together. Grab things before the other guy. These are called "massively multiplayer" worlds? Sure, there are a lot of people around, but they don't play together on a massive scale. It's like the old coin-operated arcades. Those people weren't playing with each other, they were just playing near each other. Online games reward pragmatism, selfishness, and minding your own business.
Guild Wars 2 changed a couple of assumptions about how this ought to work, and the result is a game with a radically different feel. If you hit a monster, you get full XP for it, even if someone else also hit it. Even if a lot of people hit it. This means the game encourages you to jump in and help other people, even if they aren't in your group. The fight is easier and you both get rewarded as if you'd done it alone. (Including your own chance at loot, unaffected by what the other person might get.) You get XP for reviving fallen players. If you help someone fight their way to a really valuable chest or resource node, you can both loot it. The game mechanics are telling you, "Other people are your allies and you should work with them".
This results in a very different play experience. Players tend to travel around together, stomping on monsters and keeping each other alive. The crowd grows and shrinks as players move through the area or dogpile on an elite boss. I can't tell you how many times I've wound up crisscrossing the same dungeon not because a quest told me to, but because I kept encountering new groups of players and running in a pack is fun.
You don't have to stop and form a party, pick a leader, assign loot distribution, and agree on a goal. You just run with people who happen to be going your way. I've spent almost half my time with the game in these informal groups. Essentially, everyone in the world is already a member of your party.
World of Warcraft, Lord of the Rings Online, Champions Online, DC Universe Online, and all the others worked so hard to make me play with others with forced-grouping quests, and it rarely worked. I either skipped the quest or stubbornly did it solo. Guild Wars 2 has turned me into a team player without forcing anything on me. Some of the groups I've been in have been truly massive.
Guild Wars 2 is massively multiplayer, in the sense that you are actually playing with all these other people, not just running past them and hoping they don't get in your way.