In the world of Dracula, being passive can get you into a lot of trouble. If you think something is fishy, or you aren't feeling quite right, you'd better say something. Staying quiet, or assuming that everything is going to be fine, will only get you bitten by a vampire. Take Jonathan Harker, for example, who ignores all the superstitious advice he's given on his way to Castle Dracula. He also ignores Dracula's advice to stay in his end of the castle, and falls asleep in a room where the Brides of Dracula could attack him. Mina is only attacked by Dracula because she agrees to stay home by herself instead of participating in the vampire hunt. If Mina hadn't been so passive and submissive to Harker, Van Helsing, and the other men, Dracula wouldn't have been able to get to her.
Mina only becomes an easy target for Dracula when she is overly passive: she stays home by herself at the request of the men, in spite of her own better judgment.
Jonathan Harker's passivity in Castle Dracula anticipates his later emasculation at the hands of the vampire, as Dracula victimizes Mina in their marriage bed.