But of all the answers here, the one I have to agree with most was the one with the lowest positive score. Smartdread's.
The Hulk is not a chemical reaction. He is not summoned by adrenaline. Nor is he controlled.
Think of the Hulk as an emotion elemental, composed entirely of negative emotions and driven completely by them. This actually is supported by the canon, and I will explain at the end of this post.
Bruce Banner has always had seriously repressed emotions, and these emotions ARE the Hulk. The Hulk hates Banner for being weak, powerless, useless.. and Banner hates Banner for the same reason. Furthermore, ALL negative feelings are embodied in the Hulk. Fear of death makes the Hulk essentially unkillable. Fear of any entity makes the Hulk crush it. And this very same fear causes the Hulk to RAGE for being weak enough to feel it.
Now, the Hulk exists inside Banner. Raging. But he is as much as separate entity as he is part of Banner. He REPRESENTS the frustration, fear and anger Banner feels, which is further increased by Banner's inability to control him, and fear of the very emotions which he has to bottle.
Put short, the Hulk is not an adrenaline reaction. He is a self propogating, self insisting being, and also, essentially, living, sentient emotion. Banner doesn't CONTROL him so much as LET HIM GO. He cannot change back if the Hulk doesn't let him, or get smashed the hell up.
Now, I'd like to explain something.
• The Hulk can see astral beings, as demonstrated by his once seeing Dr. Strange's astral form.
• The Hulk cannot be mind controlled for long, by anyone.
• Telepathy, and mind-control, which is a subset of telepathy, are accomplished through, or with energies from, or similar to, the Astral Plane. (this is why all powerful telepaths can manifest an "Astral Form" which is their representation on the Astral Plane).
• The Astral Plane is the source of the Power Cosmic (as stated by the Silver Surfer).
• One aspect of the Power Cosmic and its abilities is transmutation and matter generation, as used by Galactus and the Silver Surfer.
• The Hulk has returned from the Astral plane and several other planes by sheer force of will, and fist.
I propose that the Hulk is a fully formed, concrete, Astral projection, connected to, and manifested by, Bruce Banner's processing of gamma radiation. This is why Banner is always angry, as the Hulk, technically a fragment of Banner's Astral form, is also a fully formed personality composed of rage and feelings of weakness and self-defense.
When the gamma radiation is drained out of Banner quickly enough, it becomes almost impossible for him to manifest the Hulk, as he cannot generate the conditions to pull the Hulk from the Astral plane. Conversely, once the Hulk is angry enough, and has been angry for long enough, he processes so much Power Cosmic (Astral Energy) that he begins to GENERATE GAMMA RADIATION, the very energy he uses to link to the Astral Plane, solidifying the connection.
This explains all the Hulk's gain in mass, invulnerability, regeneration and mutation.
However, it does bring up another question. If I'm correct, doesn't that mean Bruce Banner is a mutant with the ability to metabolize high energy electromagnetic radiation to create a connection to the Astral Plane? He's a portal?