So yeah, a lot of Pokemon weaknesses do not make many sense. So, I tried to explain them in my own goofy opinion. Obviously this is more of a joke, so don't nerd rage on me.
First-Hand fail Explanation of Weaknesses
Normal:
Fighting: Fighting Pokemon are basically a normal type Pokemon with extra physical skills. So yes, Fighting types kick Normal types' butt.
No effect: Ghost and vice-versa. Normal is too simple and ghost is too… dead? O.o Errr...
Fighting:
Flying: Well, when I think of flying being strong against fighting, I think of a Pidgey flying around a Hitmonchan's face all like, “muhaha can’t touch this!” *peck* *peck* *peck*
Psychic: I say mind over body.
Flying:
Rock: Ever hear the term, “hit two birds with one stone”? Explained...
Electric: Well yeah, flying in the air and BOOM! Also, I have read about birds dying quite often from storms.
Ice: Try to fly when your wings are frozen. Also, most birds fly south in the winter to escape the cold too.
No effect Ground: Well in battle I think flying Pokemon knows better than to be on the ground. So no effect because of constant miss.
Poison:
Ground: Well have you ever tried to poison the ground? Probably not, since it wouldn’t work and it would be pointless to. (nor would it work on a rock, but yeah it isn’t considered a weakness)
Psychic: Mind over body excuse again? Good luck with concentration while a Muk sits by you. Actually, just being around a Muk is distracting. Psychic Pokemon just can't handle the stink and the yuckiness. So their defense is lowered.
Ground:
Water. Yeah it will get all soggy and muddy. Yuck!
Grass: Throw some leafs and flowers on a Ground type, and it will be overwhelmed it was used for nature.
Ice: I think of snow and how it completely covers the ground. It is like suffocating and forgotten.
No effect: Electric. Well when lightning hits the ground in an open area, no one cares because it doesn't effect anything right?
Rock:
Fighting: Well I guess Pokemon are Masters at martial arts and can break any type of rocks. Hiiii-YA!
Ground: Well one small rock… miles and miles of ground. A rock is inferior to the ground O.o (Stupid)
Steel: Steel is usually stronger than rock in the every day world.
Water: Water can slowly erode rocks! It is a slow process, but still legit.
Grass: Throw some moss, leafs, and flowers on a rock, and watch it die of embarrassment.
Bug:
Flying: Birds eat bugs for breakfast, lunch, and dinner.
Rock: Poor little buggies get squished from mean ol’ rocks.
Fire: Lol I think of burning ants with a magnify glass.
Ghost:
Ghost: Ooookay here we go… this is a tough one. Maybe they are weak from too much ghosty energy? Like ghost explosion? O.o
Dark: Well maybe since a ghost’s energy is in a pure form, dark can corrupt it? Or maybe they are just scared of the dark? >_> Well most ghost Pokemon already look pretty “dark”.
No Effect: (Normal already covered)
Fighting: Well try to punch a ghost and see what happens… Spoiler: NOTHING.
Fire:
Ground: Well you can put sand on a fire to put it out.
Rock: Good luck trying to set a rock on fire. Smash out the fire with some rocks?
Water: Obvious
Water:
Grass: Grass needs and absorbs water.
Electric: If you have any questions about this, go swimming during a thunderstorm. Don't actually do that...
Grass:
Flying: They are going to eat you! (Wait, isn’t that bug?) Well still a bird sits on a tree… a tree does not sit on a bird. >.>;
Poison: Poison kills plants D;
Bug: Well bugs USE grass and nature for their benefit. I think it is effective, since it probably annoys the hell out of grass types.
Fire: Obvious
Electric:
(Needs to be weak against more types)
Ground: As discussed earlier. Lightning isn't going to do anything to ground.
Psychic:
Bug: Did they just need to make things more balance? Well maybe psychic Pokemon are just really grossed out by bugs. I know I am (unless they are cute :3).
Ghost: Well BOO! Ghosts probably interrup with psychic abilities, and psychics are more sensitive to spirits/ghosts.
Dark: Probably mind/energy corruption again.
Ice:
Steel. Steel can just break ice like glass.
Fighting: Fighting Pokemon will chop up some ice into ice sculptures! Honest it is the same case with steel. Ice can be very fragile. Especially to a fighting Pokemon who seems to be able to punch/kick through everything.
Rock: I believe it is the same as the above two weaknesses. Throw a rock at some ice, and watch it shatter!
Fire: Obvious
Dragon:
Ice: Well dragon has to be weak against something right? So um… freeze it…
Dragon: Too powerful for their own good? Dragon battle... GO!
Fairy: I think of it as mythical creature vs. mythical creature. Are fairies simply too cute for a dragon to take? Are dragons just allergic to fairy dust? It is hard to draw a conclusion until we know more fairy type moves in X and Y.
Dark:
Fighting: Like superheros beating up the evil bad guys.
Bug: Bugs are too pure and (stupid) to get over taken by dark. So Dark Pokemon just can’t handle the pressure.
Steel:
Fire: Melt that steel into a puddle!
Fighting: I guess it is true. Fighting Pokemon are Masters at martial arts. POW!
Ground: Going back to where it came from!
No effect: Poison. No way you can be poisoned if you completely made out of steel. o.o
Fairy:
Although nothing official has been released, I found a source that said, "Fairy-type Pokémon themselves have been hinted as being weak to Steel- and Poison-type attacks."
Steel: Way to strong and heavy for a little fairy.
Poison: Too fragile to fumes.
(I really thought Fairy was going to be weak against Dark or Ghost.)
Bonus: I don't think Bug and poison should be a type. Also, I think ground and rock should be combined. As I said before, Electric type needs more weaknesses!!
So, I know everyone won't agree with everything I said above. Especially since some were a complete guess since they make NO SENSE! Feel free to post why YOU think some types are weak against other types!