I quickly turned to the reviews once I finished watching this movie because I don't want to feel I'm alone in being brokenhearted. Ghibli films always have their own surprises, but this one is quite beyond what I expected.
In a way, the protagonist did choose his pride that led him and her sister the miserable life and for some it might look like a totally foolish decision. But whatever he chose, either way, war only brings misery. So I understood his choice, and got emotionally swirled watching and every single thing he did for his sister.
Saying Grave of the Fireflies a "Sad film" is an understatement. It's heart-shattering. Don't expect to feel happy and entertained.