If a man learns about something that vitally affects his girl, but decides not to tell her because it would only distress or frighten her and he reckons he can deal with it himself, her not-knowing will inevitably cause her to walk obliviously into mortal peril from which she will need saving. But it's OK, because after she's saved from the danger he put her in she will certainly swoon gratefully into his arms, rather than smack him over the head for being such a stupid arrogant wazzock and tell him to get out of her life.
This trope is way older than the movies, but it has annoyed me since I was a very little girl. Look, if there's a flesh-eating monster in the lake or an axe-murderer in the woods, just TELL her already, OK? Don't just say 'Darling, I'd rather you stayed in the house on this lovely sunny day' without giving her the reason!