This is like… this shit is your soul. You can suffer dragonbreath and liquid nitrogen and suppressive fire as long as this meter is non-empty. it’s the ghost in the machine, the umbilical cord between the tangible game-experience and the heavy-breathing gamer. For you the player, Life represents the arcade era's final precious quarter, the license to game.
So how do you get more of it? I mean, yeah, a game moves along and your character amasses life experience, they’re gonna get better at stuff. The only path to mastery in reality is through gaining new experiences and learning from mistakes. Swinging a +2 Bastard Bloodsword once might not make you any better at swinging it; but swinging at 100 times at different body parts of various creatures will probably qualify you to use the weapon more effectively (kudos to Skyrim for using a level-up system that actually depends on using the relevant skill). Or if you accumulate 200km mileage of jogging back and forth across a low-gravity space station, you’ll probably be able to run more steadily and quickly.
But how - HOW?? - does taking a bullet in the stomach make you better at doing that? To generalize, how does a video-game character “gain health”? Does their hide thicken? They have more blood somehow? Or like… weird tumours all over the place which absorb damage? I mean, sure, characters get better at dodging and blunting attacks, but that usually translates into a dodge skill or armour stat. We’ve been over why a healthpool certainly isn’t mere pain tolerance, because when it runs out, you die. No amount of experience-won pain tolerance is going to make a twin-kidney katana Delta-strike cause any less... death. It is something I’ve wondered about since I first played Dungeons & Dragons in the eighth grade. You can make up a story about how literally every other element of leveling up works out in an RPG; XP represents hard won respect from the elder wizards in town, who bestow you with new arcane spell knowledge, or with time, your bond with your steed improves and it is willing to push itself harder for you, or your hard work in the forge has caused your wrought-iron goods to come out sturdier and more valuable. But your life pool increasing? More arrows in the eyeball before dropping dead…? My DM told me once he’d dreamt the answer to the riddle of HP growth, but completely forgot upon waking up.