What makes Lucario weak to Fighting-type moves?

Lucario partially are Steel-typed, which creates a weakness to Fighting-type moves. However, since their other type is Fighting, why are Fighting-type moves not at least counted as “Effective” rather than “Super Effective”? What causes them to be weak against these moves and why? I want to know the logic behind this.

Pleaselog inorregisterto add a comment.

Lucario is part Steel-type, which is weak to Fighting. Fighting does not resist Fighting. This means that when Lucario is hit by a Fighting move, it hits its Steel typing for supereffective damage. While it is true that many types resist themselves — Fire, Water, and Psychic, for example — it is not a rule that a typemustresist attacks of the same type. Hence why Normal does not resist Normal, and Ground does not resist Ground. While itispossible for a Pokémon’s typing to nullify a weakness — like how Rotom-Wash’s Electric/Water typing makes Electric attacks hit neutrally instead of supereffectively — this is not the case for Lucario.

TL;DR Lucario is weak to Fighting because it is part Steel-type, and Fighting does not resist Fighting.

Well, dragon and ghost types are super-effective against themselves, so the deal between fighting not being not very effective or does not affect fighting is simply what game freak wants it it be like.