From Andy Ihnatko, Boston, MA:
1) Marvel Comics is a "shared" universe. The Fantastic Four, the Avengers, Spider-Man, and plenty of other of folks in tights are all headquartered in NYC and cross paths often, as dictated by creative genius. Or, as is more likely these days, by the publisher's realization that if you devote the entire summer to a single mega-event store that spans all titles, fans of (say) the Fantastic Four will have to buy six comics a month just to figure out why they're all teenage girls all of a sudden.
In fact, the premise of "The Avengers," -- one of Marvel's longest-running series -- was that the Hulk goes on a rampage and a bunch of heroes (each with their own comics) get together to try to stop him. But he was being magically controlled by Loki, nemesis of Thor. When that gets cleared up they all gang up on the true villain and promise to work together as a team in the future. And then in the very next issue, the Hulk has a hissy fit and quits the team, very much setting the table for the next 40 years of the character's continuity.
2) "The Super-Soldier Serum." One of the Top 5 Maguffins of the Marvel Universe. Scrawny 4-F Steve Rogers tried to enlist in the Army at the start of WWII but was rejected as unfit for service. But he's really keen on killing Germans, so he volunteers for a top-secret Gummint program researching ways to accelerate the human body to the very peak of its potential. Rogers is injected with The Latest Random Guess, which instantly morphs him into (ironically enough) the absolute Aryan ideal of blond-haired, blue-eyed, tall, muscular humanity. A Nazi spy witnesses the experiment. It never occurs to him that hey, you know who's REALLY interested in commanding an army of Aryan-idealized soldiers? So he immediately shoots the scientist dead. The secret of the SS Serum dies with him, along with whatever chance the spy had of being named Covert Nazi Operative of the Year at the big annual awards luncheon in Heidelburg.
The Gummint, tantalized by this freak, apparently totally undocumented success, has been seeking to replicate the serum (or at least just its effects) ever since. It's the Standard Explanation Given when a writer wants to do a standard-issue "the Government is creating twisted super-freaks" story, or a "The Gummint will stop at nothing in pursuit of its own twisted agenda" story, etc.
3) The ongoing "Hulk" story seems to go through regular, cyclical phases as new writers come and go and want to realize a "fresh take" on the character, a goal which is articulated solely as "different from what the last guy did."
a) "Hulk as uncontrollable force of nature." Like a tornado in purple pants. Stories are usually about the people who encounter him and deal with aftermath. Hulk is a mindless engine of rage, which cannot be reasoned with or blamed. Or
b) "Hulk as brooding emo loner outcast." Hulk has normal intelligence and a calm, if embittered personality. Usually also has the same genius-level intellect as Bruce Banner. This personality is like the kid in 10th grade who fills notebook after notebook with awful poetry about how nobody understands him and it is his lot in life to e'er be on the outside looking in, doomed to be forever misunderstood, etc. Only sometimes he's manipulated into, or freely chooses to, pick up a thirty-story building and smash someone over the head with it. Then later on he broods about how people now trust him even less now that he's reduced an entire army into a vague greenish-red relish, o cursed Fate, why does She mock him so, etc. Or
(Still substantially less annoying than an _actual_ 10th-grade emo kid. Last year, the Hulk brutally crushed every hero in New York City, demanded that the entire city be evacuated of all civilians, and turned Madison Square Garden into a gladiatorial arena where he forced the hero community to battle each other to the death for his amusement. But at least he's never posted an earnest, teary webcam video of himself to YouTube)
c) "Hulk as the curse of Bruce Banner." Bruce tries to find cure/tries to find an accommodation/tries to mitigate the damage/is forced to face the music for the Hulk's actions. Sometimes expressed in true Jeykll/Hyde fashion.
Hulk is one of those characters that Marvel has a very hard time with. In 40 years, they've never locked down that simple sentence or two that defines the character, unlike the Fantastic Four or Captain America or Iron Man, say. So they keep trying out new gimmick after new gimmick. And then, having exhausted every possible trick, they start the cycle all over again. I don't think it's ever been a high-selling title.
It's possible that like Dr. Doom, the Joker, or Burt Reynolds post-"Cannonball Run II," the Hulk is best used as a character who makes rare and exceptional appearances in other people's stories instead of trying to carrying the whole show himself.