No, some of the dungeons are sidequests. Most (but not all!) of the leveling dungeons (15~49, 51~59, 61~69) are unlocked by, and required to continue, the main story quests, but for the level 50, 60, and 70 dungeons, only 1/3 ~ 1/2 of them are covered by the main story.

With that said, none of the sidequests ones are hard to unlock. They're almost all talk to NPC, visit the rough entrance of the dungeon, now you can do the dungeon.