To answer this, I would first have to consider what the issues are in Much Ado. There are issues such as gender roles, class roles, trust and distrust, love and marriage, reality versus what appears to be reality, and many more!
In his plays, Shakespeare uses a variety of tools to make his point. One thing he does to illustrate the issue (or theme) of trust and distrust is to show us two couples - Hero and Claudio, and Beatrice and Benedick. At the beginning of the play, Beatrice and Benedick do not trust each other, have no intention of trusting each other, and furthermore, won't ever trust anyone of the opposite gender. While this is happening, Claudio and Hero fall head over heels in love with each other and all is right with the world...until Don John comes in and tries to ruin everyone's lives! So then Claudio distrusts Hero - to the point that he publicly shames her at their wedding - but Benedick and Beatrice come to love and trust one another (yes, through the trickery of their friends, but hey, it worked!). So Shakespeare has inverted those characters to show us how issues like trust and mistrust can be turned upside down and all around in life, as well as on the stage.
Give the play another read and look for characters that are antiethical (opposite) to one another - Don Pedro and Don John, Leonato and Dogberry, Margaret and Ursula, etc. See how they illustrate the themes within the play, and be sure to check the links below for more information.