Thor telling Mjolnir to protect Jane
Maybe that's what made her worthy? It was Mjolnir trying to protect her
To be clear, I'm not someone who has a problem with it, just something that occurred to me when I had it on earlier.
Also, while I am a fan of comics, I'm a casual fan, because I don't have a ton of extra time or money to devote to it. So that's not a storyline I've read, so I don't know if it gets really explained there.