RabanoDOOM

Fairy is weak to Steel and Poison because they're industrial, factory-ish types, which conflict with the magical and nature-y Fairy type. That's also why Fire resists Fairy, since Fire could also be thought of as a sort of industrial, factory type. Dragons are weak to ice because they're reptiles. Reptiles are cold blooded and need heat to survive. Psychic is weak to Bug/Dark/Ghost because they're all common fears, and fears inhibit focus. Grass is weak to Flying because birds tend to ravage crops. That's why we have scarecrows. Normal and Ghost types are immune to eachothers because normal people don't believe in ghosts or the supernatural. Poison's weakness to Psychic is weird, but if you can read the future predict that something is poisoned, you can prevent yourself from being poisoned, right? Steel is weak to ground because earthquakes are very good at destroying things like bridges and buildings. Dark is weak against bug because bugs can see in the dark. Personally, I think that Steel should be weak to electric, since it actually conducts electricity BETTER than water does. I also think Poison should be strong against water, but that's just me. I have no explanation for Dark against Ghost.