Levels 1-15 of your first class are the only levels required to be solo content by the nature of the game. Even then you still have FATEs, which give better reward to those who group up. After you reach 15 on your first class, there is dungeon content you can do with three other players at a time. Yes, there are still story/class quests that you can only do solo but having a little solo content mixed in with party content is not a bad thing. Then, at top level you really need to either have a decent free company or linkshell to finish the main story quest or do any of the end game content thereafter. Sure, you could just pick-up-group your way through it using Duty Finder, but here's the beauty in XIV's system: You're not being forced to do it one way or another. You can choose to find a free company to play through content with or you could find a party through Player Search or shouting in zone or you could even just use Duty Finder.