Bug>Psychic is because Psychic is about the mind and thinking. Bugs rely on primal instincts to do things, so they are able to outdo those who have to think first. Or at least that's the explanation I've heard, I like the fear thing better though
Bug>dark is because Dark types are about dirty fighting and tricks. Again, primal instincts thing. Bugs don't think very hard and are thus unable to be tricked in the normal manner (I.E. Bugs are too dumb to be tricked XD)
