Why do people think only the first 40 minutes of Full Metal Jacket are good?

All of the film is good, its a masterpiece and one of the best war
films I've seen. Sure, I tuned in quickly into the film, it was
entertaining, but I enjoyed all of it. So... what makes the rest of
the film not as good as the first 40 minutes? The ending to Full Metal
Jacket was really good.

BQ: Common complaint about a film you like that you disagree with?

lol at suggested cateogry Cleaning  Laundary...