TL/DR version of the below: I think Thor's story arc is the best thing about Endgame.

"What if the good guys lose?"

According to Robert Downey Jr, this is how Kevin Feige first began the explanation of the idea for 'Infinity War' and 'Endgame' to the cast.

You see, audiences have certain expectations when they go to see a superhero movie and an action movie in general. They expect that the good guys will come through at the last second and prevent whatever disaster the villain is trying to create. We've seen this happen 1,001 times, including in each of the 18 MCU films before Infinity War.

So the idea for 'Infinity War' was to subvert audience expectations by making it look like the good guys are going to pull off the usual "win at the last second" and preventing catastrophe only to have it go exactly the opposite way by having the bad guy pull out a surprise win at the last second with the good guys completely staggered and boggled by the reversal and not knowing how to handle a decisive defeat.

This subversion of audience expectations WORKED, in my one screening of "Infinity War" on April 27, 2018, at least half the audience expressed anger, both at the fact that the movie ended with the bad guy winning, and that half the good guys died as a result of that win.

The only other major blockbuster movie I can think of that had a similar 'bad guys win' ending and which alienated at least the audience at the time was "The Empire Strikes Back" in 1980, a movie which, I can remember, also had audience members yelling their disapproval at the screen when the movie was over.

If "what if the good guys lose?' was the concept for "Infinity War", then "How do the good guys respond to defeat?" was the idea for Endgame.

The main theme of "Endgame" is how the good guys respond to defeat, every character responds in a different way. I could go on and on talking about how the various characters respond to defeat, but my interest now is Thor because, to me, Thor's story arc is the most interesting.

So let's recount Thor's story.

To fully appreciate what happens to Thor, you have to back to the beginning of his story arc which is in 'Thor Ragnarok'. If you recall, the movie opens with Thor a prisoner trapped in some kind of net by a fire demon or whatever he was. In this scene, Thor is not at all worried or scared, he is supremely confident because he is 1500 years old, and he's been doing this hero thing for nearly that long and if there is one thing he is sure of, it is that he always wins.

So, he calls his hammer to him, breaks free from the net, and the movie begins playing Led Zeppelin's 'Immigrant Song' as he attacks and destroys an army of apparently thousands of bad guys without even exerting a muscle or breaking a sweat. For Thor, this isn't a great victory, it's just Tuesday, he's done this thousands of times, and it has almost always been just as easy.

But this victory at the beginning of Ragnarok is his last victory for a very, very long time. On his next quest, he will suffer nothing but defeat after defeat after defeat, each one bigger and more devastating, with more severe consequences, than the one before.

Thor Ragnarok is really little more than Thor getting his ass kicked over and over and over again and ultimately losing in the end. Infinity War is not the first MCU film where the good guys lose, Thor Ragnarok is.

First, he loses his hammer when it is smashed by Hela. This is not just the loss of a weapon, to Thor, his hammer is like an extension of himself, the loss of his hammer is humiliating, and I would go so far as to say, emasculating, yes, the loss of his hammer is almost like a symbolic castration.

But then he gets trapped by the Gamemaster and can't escape, then he gets his ass kicked by the Hulk. When he finally gets out and goes back to Asgard he finds that all of his power is useless to defeat Hela, and in the end, he is forced to destroy his own homeland just to defeat Hela. He also loses an eye in the battle.

So, where are we? By the end of Ragnarok, Thor has lost

1. his hammer

2 his hair is cut off (this may sound trivial, but to a Viking warrior, long hair is a sign of honor of nobility, of victory, having it cut off is a sign of disgrace)

1. He has lost an eye

2. He has lost his home

But...he is on his way to a new homeland, bloodied but at least he's still standing, and even if he lost, he did put up a good fight, so maybe things are on the way up. Except....nope.

Immediately, his ship is attacked by Thanos, who drags Thor around the ship by his hair like a ragdoll. And Thor is forced to watch helplessly as first Heimdall (his best friend) is murdered, then Loki (right after he finally seems to have succeeded in making amends and decisively turning Loki to the good side) is murdered, and finally half of all the remaining Asgardians.

It is this point that he is rescued by the Guardians and goes on a quest for a new hammer. He breaks down into near tears in front of Rocket after Rocket reminds him of all he lost.

The movie ends with Thor failing to kill Thanos and Thanos performing the 'snap', for which Thor blames himself.

Endgame begins with Thor being broody, Tony Stark asks 'what's up with him?' and Rocket responds 'he thinks he failed, but then there's a lot of that going around, isn't there?" Rocket is the only one who understands.

Then, the heroes go off on a quest to kill Thanos, a quest that turns out to be pointless.

Flash forward 5 years, Rocket and Hulk visit Thor in New Asgard. Valkyrie tells them that they never actually see Thor except when he is out for 'supplies' i.e. more beer.

When Rocket and Hulk she Thor's condition, they are shocked and worried. Both express concern, but Thor insists everything is fine and he doesn't need help. When Hulk says he needs Thor's 'help' it triggers a reaction 'why don't you ask those people down there what my help is worth? The ones that are left.' We see a hint of what is really bothering him.

A common complaint is that Thor's condition, which appears to be the depression and depleted self-confidence that comes from the worst stages of PTSD, is played solely, or mostly, for laughs. I don't think this is true.

There are only two jokes made about Thor's condition in the entire movie. The first is by Tony Stark, who calls him 'Lebowski', which is best interpreted less as an attempt to make fun of Thor than as the kind of the kind of gentle good ribbing he does to all of his friends (remember, he previously called Thor 'Pointbreak' in the first Avengers movie). The second instance is when Thor asks 'do you know what is running through my veins?' and he replies 'Cheeze Whiz?' This is best interpreted as an attempt by Rhodey to try to lighten a tense situation with a little humor.

Other than these two moments, when his friends are lightheartedly poking fun at him, in the same way that they always have, I defy you to name a situation in the movie where it is played entirely for laughs, or portrayed as something other than troubling behavior for a 'great warrior' that has all his friends deeply concerned about his well being

This has already gone on too long, I'll now end and let you guys raise your objections.