Quote Originally Posted by willpell View Post
It can't possibly be a conscious effort to resist, because if someone casts a Charm on you and you fail the save, you don't know that you made one. It must be reflexive, automatically triggered by the targeting of your mind with hostile magic; it's essentially an immune system, and while you might be able to do things like intentionally exposing yourself to more germs, you can't actually make yourself succumb to a disease. Your body won't allow it, and the same goes for Will saves - they're autonomic defenses with which your non-conscious components will protect the whole of your self even from you.
Charm Person is also Enchantment and Mind-affecting. You don't know you failed the save because *Your mind is altered by enchantment magic*, so you forget you made one.

There's a section in the rules detailing voluntarilly failing a save, and another about lowering your spell resistance. Creatures know about these things in-universe.