GW2 is great because it fosters cooperation instead of competition. CS:GO, LoL, GTA:O, Rocket League, Dota2, Starcraft, etc, etc, etc. all are built around some level of competition. Even cooperative games will encourage competition through leaderboards or by giving better rewards to higher-scoring players. Everyone is fighting to be the best and you end up with a game where even your friends are your enemies and rather than pulling you up they just climb right over you in an endless struggle to reach the top.
GW2 is different, though. In GW2, your success never comes at the expense of someone else or vise-versa. It's almost never a bad thing to have someone else there fighting with you, and it's almost always better to help someone up than to push them down. Why spend your time trying to be better than your teammates? When you beat a boss there's no magical scoreboard that divvies up loot based on how much each player contributed; if you win, you win, and if you lose, you lose. Instead, if there's someone who's struggling or holding back the team, help them out. Their success is your success.