- The normal type is not supereffective against any types, but is immune to ghost type. The reason is comes from the belief that ghosts and organic beings cannot interact with each other.
- Fire is supereffective against steel because it melts at high temperatures.
- Water is superefffective against earth because it reduced it to slush while for the rock type is because of the erosion process.
- The Electric type is supereffective against flying type, but it does not make much sense because in real life the birds have less probability to get hit by thunders becase lightning mostly tends to hit the ground. The steel type are resistant because they are made of metal, so lightning does not harm them. It has not effect to ground type due to the grounding process.
- The Grass type is supereffective against water due to the roots absorbing water, earth because tree roots can literally break the ground, and rock for the same reason.
- The Ice type is supereffective against earth because it makes the earth sterile, completely unable to produce plants. Againt flying because it limits their moviments and the wings of birds.
Note: For Dragons its because they are similar to all the reptiles that have cold blood, so they tend to show up when the temperatures are high enough.
- The fighting type is supereffective against a normal type because a trained athlete can easily defeat a normal untrained human, its also against rock, steel and ice because it is said that karate masters are able to break these materials with their hands.
Note: It is supereffective against Dark types because it means evil, and the fighting is associated with good and in most stories good triumphs over evil.
- The ghost type is immune to fighting because they pass through them due to them being non-corporeal.
- The poison types is supereffitve against grass because pesticides are literrally made of poison, for the fairies its because most faires commonly live in pure and peaceful forests, and poison can ruin and kill them.
- Steel is immune to poison because steel does not get ruined by poison, metal blades can be enbedded with poison.
- Earth is supereffective against poison because...... if a vial of poison falls into the ground it cannot be used anymore. For rock its because earthquakes can break rocks, flying birds are uneffective by earthquakes since are earth based.
-Flying is supereffective against grass because they use grass to make a nest, fighting because if judo is under a pigeon, he will receive something. Birds eats bugs.
- Psychic types are supereffective against fighting because the mind means more than the body, for poison because an intelligent person obliviously avoids being poisoned,
Note: Psychic has not affect to dark types because some fears cannot be defeated.
- Bug types are supereffecite against grass because most insects eats plants, dark types because they have great sight under the dark, and psychic because some insects are the phobia of many persons.
- Rock types are supereffective against fire because rocks can be used to stop a wave of fire, massive rocks can also be used to neutralize ice, birds and bugs.
Note: The ghosts types are superefftive against psychic because they are a common fear, against ghosts because they can interract with each other and not organic beings.
- The dragons are big and can harm eachother, they cannot harm fairies and are heavily harmed by them because in most fairy tales dragons are killed by magic speels, R.I.P Hydregon.
- Darkness as a fear is supereffective against the mind [associated with psychic], Gamefreak thought that the darkness can repell ghosts, which is questionable since that is supposed to be the light.
- Steel is supereffective against the rock because its the better material, ice can be broke by steel equipment, and fairies because in the Britannic Mithology stell is used to fight and repell fairies/magic beings.
- Fairy is supereffective against fighting because trained judo's can be easily stopped by magic, for dark due to the good prevails over evil thing, and dragons because they are killed by magic in some fairy tales.
- The others like Water is supereffective against fire or Ice against grass are not needed to explain since its oblivious.
Final Note: Credit to Pokémon Millenium, translation done by myself.