By completing quests, you get XP: gather enough, and you will level up, allowing you to tackle even more difficult quests. (perfectly-nintendo.com)
You earn money by doing quests, turning in quests gains you rep, more rep means more difficult quests. (kitsuga.com)
Before you can face it, you'll need to prove yourself to your guild and increase your hunting rank by taking on increasingly difficult quests. (gamesradar.com)