I've been thinking about this movie constantly since I've watched it and I've come to one conclusion: most people don't understand the character writing in this movie.
I've seen many people criticizing Seita's actions as conceited and selfish, but they're missing the bigger picture here: that Grave of the Fireflies is at its heart a criticism of Japanese culture during and after World War 2, and Seita represents Japan.
Seita is undeniably arrogant and selfish in this film; it's the source of frustration among many critics who claim he could've done more to save her and that the situation they ended up in as completely his fault for being foolish. While they're absolutely correct, this is intentional. It's a metaphor for what happened to Japan during World War 2 that led to their defeat.
Japan was prideful and arrogant before the bombs were dropped on their cities. They were basically defeated before the Fat Man and Little Boy were ever dropped on the cities. Japan had been advised by the United States and other countries to surrender; that they were prolonging an inevitable defeat and that continuing the struggle would only result in more deaths and more pain. But Japan refused to listen, too arrogant to let go of their pride and give in for the better of its people and future, feeling that the rest of the world was looking down on them in disdain.
This is exactly what happens to Seita and Setsuko in the film. Seita could have let go of his pride and just dealt with his aunt, despite her being cruel to them both. But he was arrogant. He couldn't accept the way she looked down on them and treated them like a burden, but she didn't kick them out. She gave them an option that Seita refused to accept purely because he was selfish and prideful. Again, it's consistent with the wider metaphor. It isn't irrational; it's deliberate political commentary on what Japan was doing during that time period.
Eventually Seita ends up homeless, still too arrogant to go back and accept his aunt's offer (a metaphor for the surrender the United States offered Japan before dropping the bombs) when he comes across her. He completely ignores her and runs away. Just before the bombs dropped on Nagasaki and Hiroshima, the United States offered an ultimatum to Japan: surrender peacefully now or we will end this war for you. Japan, in its continued conceit, completely ignored the opportunity and paid the ultimate price for it. It suffered complete, devastating defeat that lasted long after the war ended. This is what the later part of the film represents.
Seita had the opportunity to salvage what little he had left, to let go of his pride and give in for the better of his dependent, Setsuko. But he ignored this opportunity when presented with it, still too arrogant to give in. Because of this, he's unable to stave off the inevitable and Setsuko dies, and he has no one to blame but himself. Once more, it's consistent with the metaphor and perfectly representative of the end of Japan's involvement in World War 2.
At the end of the film, we see Seita lying in a lobby of some sort, completely defeated and lifeless. Again, this is a great representation of the state of Japan after surrendering. They refused to listen and let go of their excessive pride, which ultimately resulted in a crushing defeat that was their fault alone. It was a devastating defeat that would take decades to fully recover from. But at the end of the film, Seita never recovers from it. That represents the end of an era for Japan. A slow, painful death that the rest of the country and world has to help clean up in frustration. That's what the men with the brooms at the very end represent. They seem annoyed and disappointed, which is more of the director's own take on the entire Japanese involvement. (That one is just my interpretation, I don't want to have people thinking this is 100% Takahata's intention. It's just what I believe he was going for.)
While it's true that showing the ending scene at the very beginning of the film somewhat reduces the shock of the ending, this is again completely metaphorical. Japan knew they were going to lose the war. The rest of the world knew they were going to lose the war. Everyone knew. That's why the ending is shown at the start of the film. The ending was not meant to be shocking; it's a brilliant framing device that completes the overarching historical parallel the film is going for.
With that, I think I've aptly explained what Grave of the Fireflies is really all about. While some may find the characters' actions frustrating, it's clear when you look at it from the perspective of history that it was all very intentional. Watch the film again from this angle; it's eye-opening how much the film changes.
