Ehhhhhh… well, the thing is, I used to go by the description given by one of the trainers in the Blackthorn City Gym way back in Gold and Silver (‘cause, y’know, you’d expect Dragon Pokémon trainers to have some idea how Dragon-types work), and what they said was that Dragon Pokémon are “Pokémon that overflow with life energy,” or something like that. Dratini’s assorted Pokédex entries have some similar lines. So if you’re okay with some abstract “life force” being a real thing in the Pokémon world (which seems more or less fine), then we could understand Dragon-types as Pokémon who have access to a sort of internal “wellspring” of that power, granting them perks like long life and rapid healing. This sort of fits generally with the holy status of dragons in East Asian mythology, the large number of legendary Dragon Pokémon with load-bearing positions in the Pokémon world’s cosmology, and whatever the hell the Dragon Force in the Victini and Reshiram/Zekrom movie is supposed to be. Dragon-types’ attacks are strong against each other because Dragon attacks are among the only things that can directly attack that energy source and overwhelm it. Steel-types resist Dragon attacks because, being partly mechanical, they are less reliant on life force than most other living things. Ice attacks… honestly I’m unclear on this, but in the real world a lot of processes that are essential to life are slowed down by cold, so maybe in the Pokémon world life force itself can be slowed and congested by extreme cold?
The reason this suddenly becomes more complicated is that, as of X and Y, we now have Fairy-types, and Xerneas gives us fairly concrete reason to believe that it’s Fairy Pokémon who are most closely associated with life force, not Dragon Pokémon. And you can maybe make some vague hand-wavey suggestions that get around that, like saying that Fairy Pokémon can manipulate and master life force while Dragon Pokémon can only tap into it by instinct, so that Fairy-types can block Dragon attacks effortlessly while also damaging the Dragons’ connection to the source of their power. When I start to do that, though, I become worried that I’m just defending my own existing ideas rather than looking for the best possible explanation, and it also seems like Game Freak’s own ideas about what the Dragon type is have evolved since Gold and Silver – I mean, it’s hard to imagine Druddigon as holy, or having a special connection to some abstract life force. So I don’t quite know.