I've already discussed the differences between burnout and GM's block over here, so let's discuss some causes.
Critical Players. Some players just like to complain or give a number of 'constructive comments' which might either be more negative than they realise or you might not be emotionally equipped to handle.
Perfectionism. Some GMs seek out a perfect game where all of the players experience Flow, just the right amount of Out of Character / In Character Bleed, and are perpetually enthralled by the game. Any session that doesn't meet these perfect requirements thus normal sessions provides no joy for them.
Lack of Recognition. It's human nature to start taking the good stuff for granted. A lot of players forget to recognise their Storyteller's efforts and even if they do it relatively frequently, it may not feel like enough if the GM is spending half a dozen hours a week on game preparation. There's also the chance the GM just can't take a compliment, either deflecting it or ignoring it, which makes it impossible to recognise their efforts.
Inadequate Repayment. It's essentially a volunteer role with no hope for reimbursement which can't be put on a resume. There may be travel costs, resources costs (printing, dice, pencils), system costs (books, which aren't cheap), miniatures costs in time (social events missed, entertainment time diminished due to game preparation, cleaning up after hosting) and sometimes even food costs (where the host provides food to all). As a general rule, the GM sinks more money into the game than the players.
Player Entitlement. Some players are especially demanding. They expect the GM to supply all the resources, complain if the GM doesn't have the supplement the player wants to use and even offer constructive criticism about how the GM doesn't have enough minis yet aren't prone to offering to pay for any of it ... even if the GM hasn't any more money than the player in question! They might also expect the GM to always be available to run a session (no personal leave allowed) or in games with a significant downtime element (like a LARP), they might expect the GM to be constantly available to them full stop.
Lack of Work. Some play groups want the GM to be a largely invisible force who sits back and waits for the players to call on them. In other cases, the GM really wants to work on clues, props and investigations while the players want to romp through pre-made dungeon crawls which leaves the GM feeling bored and underutilised.
Tasks With No End. Enemy stats created ... new enemy stats needed. Floorplan / map / clue path generated ... new floorplan / map / clue path generated. If you can wing it, then at least some of these tasks can be completed and then left to sit for awhile. Otherwise you keep on keeping on the same tasks with a vaguely fresh face.
Impossible Tasks. Some GMs also sometimes set themselves up to do massive undertakings - running 30 player LARPs single-handedly, running five campaigns a week which rely on pre-planning, or even doing both simultaneously!
Problem Players. Many GMs take on a player's problems as their own personal responsibility to solve. They convince themselves that they can do what highly paid and trained Managers and Consultants can't do to their paid employees ... change them to suit the role at hand. Only the player can change their problems and only if the players actually think those traits are problems.
Incompatible Demands. Some players needs stand in opposition to the needs of other players or the GM. Trying to integrate a player who adores gritty realism, historical accuracy and intellectual pursuits into your superheroes dungeon crawl in the long-term is difficult at best. Sometimes it just won't work.
Bureaucracy. Some GMs do all the paperwork, note taking, summarising, experience point tracking, damage tracking, rules tips and character sheet updates for their players. This might be because the players don't want to do it or because the GM thought they'd be helpful and took on too much work.
Value Conflicts. Some players / GMs see the other side as being subservient to their needs. The player sees the GM as a free entertainer whose every effort should toward player enjoyment and whose only reward should be a player's enjoyment. Some GMs see players as actors who must obey their every directorial whim. Sometimes the values conflicts involves, say, a historical reenactors' desires for accuracy versus a person of colour's desire for escapism from a racist world.
Meaninglessness of Achieved Goals. Once you get cynical, it's hard to find meaning in constantly being expected to provide entertainment for other people. There's no promotions, no pay rises, and no method of tracking your progress. Are you getting better or worse? This can be a particular problem for sandbox GMs who can't even grin at a job well done and a campaign well finished. What are the Key Performance Indicators of a GM, anyway for a GM anyway? Once you start getting burnt out and cynical, the goal of "give your players a fantastic time" starts getting a derisive snort and a "What makes players so special? Why is no one trying to give me a good time?" Once that sets in, the main reward is gone.
Role Ambiguity. What is a good game, anyway? How much are your players really enjoying it? How many liberties can you take with a campaign to ensure your own enjoyment before you become a dud GM? What are your obligations? Where can you ask for help? Confusion leads to questions leads to self-doubt, lost time and more expended effort as you mull it over.
Workload. Too much or too little are both problems. If you have too much to do, you become naturally exhausted by your efforts. If you have too little, you can suffer bore out where you start detaching from the game which can make the game seem like wasted time.
Regrettably, one of the first reactions to the start of burnout is often to intensify one's efforts. "Maybe if I put in more hours, I can make the game work again." "Maybe once this is fixed, everything will be okay." "Maybe once I try this, this irritant will go away." Maybe ... Maybe ... Maybe. Unfortunately one of the key causes of burnout is the levels of self-esteem crushing self-doubt, self-questioning and increased effort that some GMs pour into their games. It's like an exhausted swimmer flailing for a life raft and tiring themselves out sooner.
A GM's burnout has a multitude of causes. Only by recognising the particular issues affecting you, can you really start to pull yourself out of it once the dread, cynicism and sadness claws its way into your heart and brain. Yes, it's melodramatic, but having one's hobby start repulsing you is dramatic in its own right, especially when you have players scrabbling for you to keep running games.