Claudio refused to marry Hero because he thought that she was cheating on him.
Claudio was tricked, by Don John, into thinking that Hero was cheating on him with another man. He did not actually see anything. What he saw was Hero’s maid, Margaret, at a window, and he thought that it was something else. Don John arranged the whole thing to discredit Hero. They say some people are in love with love. Don John is in love with hate.
In Act 2, Scene 2, Don John and Borachio designed the insult to Hero’s honor. They would make it look like Hero was with another man, when really it wasn’t Hero at all—it was just her window.
Then Don John just had to make sure that Claudio went to the window and saw “Hero” doing the dirty deed. He would then doubt his girl. Don John tells Claudio that Hero has been “disloyal.”
Claudio, being the rapscallion that he is, decides not to confront Hero until her wedding day, where he can disgrace her before everyone they know, and tell her then that he isn't going to marry her! He calls her a “wanton” and while everyone is completely confused, he hurls insults at her.
This is, in some ways, one of the most tragic scenes in all of Shakespeare. Poor Hero has done nothing wrong, and dies a symbolic death. Her reputation ruined, Claudio believes the worst of her without reason. He was deceived, and she has no recourse. The solution that they come up with is to tell everyone she is dead, but in some ways, she is. Claudio refused to marry her, but no one else would either. Everyone will believe his accusations.
Claudio's behavior after this is just as bad. He agrees to marry a "substitute" for Hero, as if women were all interchangeable. The substitute turns out to be Hero, but why would she want to marry this clown anyway? The truth is, she has little choice. Even after the truth comes out, the smear he has done to her will follow her ever after. It may be Don John's work, but it is Claudio's fault for not believing in her goodness. He said he loved her. He should have given her then benefit of the doubt.
All of this is just much ado about nothing. They made a big deal about something that didn't really happen. However, to Hero, it will follow her forever. This play tells us what can happen when rumors get started just for fun, or because a person has a bitter heart. Don John just enjoys causing grief. He wants revenge because he can't have what they have, and revenge on his brother for being the legitimate one. Hero and Claudio have nothing to do with this, but they get caught in the crossfire. So the next time you pass along a piece of juicy gossip that seems innocent, think about who you might actually be hurting. Is it really nothing?