He's over this Marvel stuff now. Doing the Avengers films was Hell for him. A dream for some, maybe. But we'll never know what it's like to actually make it . He thinks Avengers 2 was wasted potential and both critics and fans attacked him and his film for various reasons. He is beaten, worn down. Sort of like Nolan taking a break after Interstellar was given mixed reviews. He felt burned down. Why do you think Jon Favreau passed up on the first Avengers? He knew it would be a damn nightmare making them and he would probably hate it. Even when directors make a masterpiece people love, sometimes, they still feel like a failure.

Because people a a-holes and broke his spirit.

This.
Making a movie as huge as Avengers is absolutely exhausting. I think at some point it's less about directing and more about logistics management, but everyone in the world, including the studio, is going to tell you that you're doing it wrong, this being expecially true on a sequel to a massive hit. If the suits really think they can make a better movie, they should try.
After the first one, he went back to something that he loves and did "Much Ado About Nothing," a lovely little film made on a shoestring budget with people he likes. As I recall, he talked about wanting to do something fun, because those big movies are not fun to make, and the pressure is intense.
As someone pointed out, it's not like he's living paycheck to paycheck, and maybe some forget, but A.G.E.N.T.S. of Shield is still on the air, and he's one of the creators, so he's got some TV money from that. Honestly, no one should be worried about a guy who has made a lot of money, has a giant fanbase and has plenty of talented people who want to work with him.
It's also possible that, after living in the Marvel universe for so long (don't forget that he was writing comics for them, too), he just needed to get back to other things. I'm hoping for more TV (not on FOX, please, which doesn't seem to know what to do with his stuff).

He wanted to go and create something original again. Also, he found Age of Ultron a very stressful experience and I think he actually became a bit depressed working there.
From what he's been saying he's never going back. It'd be great if he did but it'd take a miracle for him to.
I've noticed a couple of people on the thread said he was fired but based on what has been said I don't think he was. I know Mark Ruffalo said he begged Joss to come back and do infinity war or a hulk movie or anything but he said he won't.

No one walks away from a multi-billion dollar franchise like The Avengers willingly.
Whow. You're quite the materialist, it seems.
I would totally walk away from that money in Joss shoes, for the exact same reasons he gave. Joss doesnt need to worry about a full refrigerator for the rest of his life anymore. So why do s16d0omething he doesnt really enjoy ? And creative freedom is especially important for an artist.
A gentleman will not insult me, and no man not a gentleman can insult me.

The comics are absolutely wonderful. Especially season 10. And the Angel and Faith series is some of the best stuff out there.

I would disagree.
I love season 6 (its my favourite) although season 7 isn't the best but to be fair to him he was running 3 tv shows and writing the fray comic at the same time.
The comics are fun, not as good as the show but still very enjoyable. My favourite is The Chain (i think its number 5). Plus, Joss is a comic lover so i doubt he did those just for the money.
No one spends money making micro budget projects like much ado about nothing or dr horrible if their purely motivated by money.
I think he loves what he does and chooses projects based on that. Also, you can't say he signed on to the avengers film for the money because he wasn't paid very much (in scale) to write and direct it.
Plus i doubt he was fired for making two movies that grossed over a billion dollars. They are the 2 most financially successful MCU films.

I can't speak for the comics because I haven't read them, but I think Season 6 of Buffy is the best of the series. Season 7 isn't quite as good, but definitely not the worst.
But to add to what the other poster was saying, I think the main reason why Joss left was out of pure exhaustion.

Joss is one of those types who's motivated by primarily by the money.
How else do you explain the last two horrible seasons of Buffy? Or those god awful Buffy and Angel comics that came out afterwards?

Perhaps. Most people are motivated primarily by money but there are a few ,a very few, who are not. Particularly if they consider thb68at they already have enough money and are not lost in the grip of greed. I'm not sure which one Joss is.

No one walks away from a multi-billion dollar franchise like The Avengers willingly.
Joss was fired and he's making excuses for it.

i saw Joss on an interview and when asked about the split with Marvel he said they offered him a lot of money to stay. They wanted his advice on the Marvel Universe since he was such long running fan that he was considered an expert and of course they wanted him to oversee certain films and direct as well. The reason he gave for turning them down was the lack of freedom for creativity. He said Marvel has tight parameters for all their movies that he have to adhere to ,that it's almost formulaic in nature and he wanted much more freedom than they allowed. He did agree to be a consultant on their future Marvel movies so there would be consistency within the Marvel universe . I would say that there relationship is amicable. Also, I tend to believe Joss' explanation. He is dome thing of a geek who cares far more about creativity and having fun on a project than money.
I imagine Joss taking a break would be in order ar to re- charge his creative juices before taking on another project.

He was fired.