According to the Watcher, there's a reason Marvel's Hulk is so angry, and it's because he knows he's dumb. When Tony Stark met Bruce Banner in The Avengers, he couldn't resist one of his trademark quips; "And I'm a huge fan of the way you lose control and turn into an enormous green rage monster," Stark joked. It's a memorable line, and it sums up the Hulk perfectly; a creature of pure rage who gets stronger as he gets madder.
But why is the Hulk so angry? Comics have suggested countless reasons, with one of the most heartbreaking being that the Hulk is the long-repressed rage of Bruce Banner himself made manifest - with Banner heartbreakingly a victim of childhood trauma and abuse. Alternatively, another theory was suggested in Marvel's What If? #2 all the way back in 1977, and it's important because it was outlined by a cosmic being known as the Watcher. Uatu the Watcher possesses the ability to see all of time and space, and Marvel often uses him to act as an omniscient narrator - meaning his words are practically unquestionable. And his assessment of the Hulk is pretty shocking.
What If? #2 sees the Watcher observe the Hulk as he flees the scene of his latest calamity and ponders his own nature. According to the Watcher, the reason for the Hulk's rage is the fact Bruce Banner's persona lies buried within him, desperate to communicate, eager to escape, yet forever imprisoned. "You are like a lion bereft of claws," the Watcher observes, "yet forever straining to rend his prey with vanished talons." In other words, the Hulk knows he should have a genius intellect but can't access it, driving him into a cycle of frustration and rage he can't properly understand or express.
According to the Watcher, the Hulk's rage is actually directed inwards rather than outwards - it is an expression of self-loathing. This concept works quite well with some of the older Hulk stories, where the Jade Giant was typically furious and confused by the rejection of those around him, and wound up roaring his desire to be left alone - to be given peace, a place without any stimulus to force him to try to think and understand. More recent series have depicted the Hulk as an inherently childlike being; a reading that doesn't necessarily diverge from this theory, and actually adds a disturbing new element of Hulk as a child haunted by a halting understanding of the man he'll never become.
Ultimately, the Watcher is as close to the "Voice of God" as any character in Marvel Comics, meaning his interpretation of the Hulk should be seen as one of the most important and definitive. It should also be noted that while Uatu shares his observation in a What If? comic - famous for depicting potential alternate realities - this observation comes before the hypothetical scenario, taking place squarely in the mainstream Earth-616. If the Watcher is right, the real reason for the Hulk's anger is simply because he desperately wants to think but cannot, and hates himself for his lack of intellect.