I DMed a game for about seven years, and had been dealing with burnout off and on during the whole process. I'd say I started having my first troubles about a year into it, and it would come and go.
It's just a ton of work, and the older I get the less happy I am with a game that hasn't had a ton of work invested in it because the level of quality I want from a session goes beyond what I wanted when I was 12. When I was 12 or 14 we'd spend hours just rolling dice and writing numbers down on paper as the PCs waded through some near random dungeon of random monsters and treasure. But now... I want more.
I burned out hard about two years ago. I tried switching systems and it went OK for about a year and at first I was excited to run games again, but still burnout. Fortunately, one of the players has agreed to DM for a while and it's been nice to be on the other side of the screen for a while.
Lately I've been thinking of picking back up the 'main campaign'.
Things that really went wrong:
a) Players are difficult. Like in a perfect world you'd have players that always RPed consistent, intelligent, thoughtful, heroic characters. In the real world, you have players with baggage, depression, bad days, and no interest in your favored aesthetics of play most of the time because they just want to blow steam. What you want as a GM is 'Critical Role', but you'd be happy most days with a better session of 'Knights of the Dinner Table'. Some sessions you fail as a GM, they fail as players, the dice hate everyone, and nothing goes right.
b) The work effort is hard, and the level of quality I'm personally satisfied with is inversely proportional to the amount of time I have in my life to work on a pointless hobby.
c) The game can just go wrong. The party has gotten itself into a really tight spot, and for the dozen or so sessions prior to me calling a halt, they'd been in a progessive death spiral of declining resources. NPC henchmen died. Consumable resources consumed. Pets dead. Now PC's are dead with no obvious replacement candidates. Potential allies are turned to enemies. No safe haven to rest at. They are losing right now, mostly through bad decisions, because a single bad decision can turn really bad. Now I'm facing a less than climatic and heroic end to the campaign, and I don't know how to dig the PC's out of the hole they are in.
d) Writer's block. Even if the PC's do win, I don't really know what happens next. The problem with running a travel/wilderness based game is that while the journey can be fun, and the destination exciting, there is that whole "and back again" part of the story that feels like you need to handwave it away. I need to do some world building so that whatever happens, I have some options, but that gets back to the work effort issue.