One key part of the plot revolves around Don John"s framing Hero to trick his brother Don Pedro and Claudio, her fiancé, into thinking she was unfaithful. When this succeeds, those who believe her concoct another plan, to trick people into thinking she is dead. Everyone then operates based on the notion that she is dead. Because her honor was falsely smeared, she must be avenged. The newly declared lovers, Beatrice and Benedick, believe her, and Benedick (reluctantly) agrees to fight a duel. However, Don John's treachery is uncovered and the duel called off.
Claudio, although grief-stricken over Hero's death, agrees to do what her father asks: marry Antonio's daughter. Not until the wedding is it revealed that the bride is (Surprise!) Hero herself.
The harshness of this plot in many ways seems out of place in a comedy, but it served in part as a device to keep the audience invested in Don John's comeuppance. Also, because Hero is Beatrice's cousin, it is appropriate for her to have a hand in seeing she is avenged. And as Benedick has just sworn his love for Beatrice, he might very well desire to prove himself worthy by fighting the duel as she asks.
Modern audiences also often find it a bigger surprise that Hero doesn't dump Claudio, as he doubted her honor and treated her badly. But she also treasures her honor, so perhaps she interprets his actions as appropriate.