I think it's because of the addicting yet frustrating loot system that Bungie has implemented into the game. Everyone wants to get the best guns but the drops are random. You can only play once a week per character per difficulty. Combined with a great fundamental shooting mechanics system, each raid requires that all 6 people know EXACTLY what their job is on the team and it also requires immense communication between all team members. If you have 5 veterans, then there's a chance that the 6th person may be carried through the raid but still, they need solid guns and fundamental knowledge in order to beat each raid. On top of that, it's just pure fun zipping through the raid. Kicking back, talking with some strangers who have the exact same motive as you, and that is to get the raid done ASAP to get a chance at getting the best legendary and exotic guns in the game right now. I've done the raid like at least 50 times now and I'm still enjoying myself.
Additionally, each raid have secret/hidden chests that also may give you random exotic drops. For example, the hawkmoon hand cannon in my display pic is one of the more rare exotic guns in the game particularly because it's playstation exclusive so Xbox players have no idea how the gun shoots like but I've seen many Xbox only gamers viewing streams saying stuff like "damn, the hawkmoon just became one of my most wanted weapon in the game". Even if you're playing Destiny on a PS platform, the gun is quite rare to come across, only dropping from random crucible matches, hidden chests in the raid or even nightfall/weekly strikes.
So one time when I was playing the Vault of Glass raid, we were at one of the hidden chests where you "MAY" get a chance of getting an exotic drop. So the first person gets a Monte Carlo exotic auto rifle (also PS exclusive), second person gets a Plan C exotic fusion rifle and the last person gets the hawkmoon (guy who got the hawkmoon was streaming) and the whole stream went nuts because everyone knew he was waiting for the longest time to get that gun. It's just simple and fun times like that which makes this game so unique.
I've had so many great memories over the past few months just like that by playing the game, it just has that fun factor nailed.