Theory: Wario is Immortal

Abstract
Wario is immortal/invulnerable.

Arguments
"Wario Land 2 is much better than any of my previous games. Why, you ask? Because in this game, I am immortal! There is no Game Over!"
 * Argument 1: On Page 3 in the Wario Land II Manual it states

- Wario

"No Matter how much damage I take, I am immortal! Ha ha! But I am not invincible. Some enemy attacks affect me in different ways."
 * Argument 2: On Page 11 in the Wario Land II Manual it states

- Wario

"Wario may be invincible, but when he's attacked by his enemies he takes damage. Just like in Wario Land 2, this game is chock-full of laughable reactions!"
 * Argument 3: On Page 22 in the Wario Land 3 Manual it states

- Instruction manual

"You don't want to die! I don't want to die!"
 * Argument 4: On the Japanese Wario Land 4 FAQ. One of the questions is "Why did Wario become invulnerable?" Wario replies with:

- Wario


 * as his reason for being immortal/invulnerable.


 * Argument 5: After Virtual Boy Wario Land, Wario cannot die. There isn't a life system in some games and in the games where he has a heart meter, he gets blasted out the dimension. No Game Over. Even things like being burnt to complete ash doesn't keep him down.

"I thought of him as reckless. But for some reason he doesn't die. (Laughs) He's completely different than Mario."
 * Argument 6: In the Wario Land Shake It! developer interview. When Ebisu Etsunobu (Wario Land Shake It!'s producer) is asked "What kind of an image did you have of Wario?" Etsunobu states:

- Ebisu Etsunobu
 * (This one is up to interpretation, as it's not clear if he meant Wario was different than Mario in that he can't die and is reckless.)


 * Argument 7: In the Waro Land 3 Japanese commercial. Wario literally says "Thank You" to enemies inflicting damage on him, which could mean there's no real danger.