In this act of Much Ado About Nothing, Beatrice and Benedick are “tricked” into falling in love. How does it happen, and why is it so easy?
Don Pedro, Claudio, and Leonato trick Benedick into falling in love with Beatrice by letting him overhear them talking about how Beatrice is secretly in love with him. In a similar way, Hero and Ursula make sure Beatrice overhears them talking about Benedick's unrequited love for her.
The reason this plot by Benedick and Beatrice's friends works so well is that the two of them are already attracted to each other, and the animosity between them stems from resentment and hurt feelings.
The text provides plenty of evidence. For example, when Claudio asks Benedick for his opinion about Hero in act 1, scene 1, Benedick says he's not that impressed with Hero's looks, saying that if Beatrice "were not possessed by a fury, exceeds her [Hero] as much in beauty as the first of May doth the last of December." In act 2, scene 1, Benedick tells Don Pedro that Beatrice "speaks poniards, and every word stabs," implying that he is truly hurt by her insults.
For her part, Beatrice hints that she and Benedick had a flirtation or perhaps something more serious between them in the past, but it didn't end well. In act 1, scene 1, Beatrice tells Benedick, "You always end with a jade's trick; I know you of old." After yet another bout of verbal sparring between the pair in act 2, scene 1, Don Pedro tells Beatrice, "Come, lady, you have lost the heart of Signior Benedick." She says Benedick did lend his heart to her for a while, and she paid interest on it. She said she gave him "a double heart for his single one," meaning she believes her feelings for him were greater than his for her.
Once Benedick is convinced that Beatrice is desperately in love with him and she believes the same of Benedict, the two of them feel safe to admit, at least to themselves, their true feelings, even if their pride won't allow them to declare their love to each other just yet.