In the first part, Benedick soliloquises about the man who "dedicates his behaviours to love" is a fool, and the irony of becoming the "argument of his own scorn." This pre-empts the drastic change in Benedick's behaviour later in the scene. Already we see a confident flurry of long, complex declaratives, signalling a kind of gusto to Benedick's emotions: he clearly feels strongly about this issue. However, the fact that he must say these things to himself may serve to highlight his insecurity with himself. It is quite obvious from earlier points in the play that Benedick is wholly uncomfortable with his feelings towards Beatrice and tries (unsuccessfully) to hide these feelings.
In the next few lines, Benedick contrasts battle imagery such as "good armour" with softer, more romantic objects, such as a "new doublet." This serves to illustrate his contempt for Claudio's utter turnaround. His tripling in this section further emphasises both Benedick's views, and indeed his own issues. The fact that he must use persuasive techniques such as parallelism suggests that he himself cannot truly bring himself to believe his apparent viewpoint.
Benedick ends with a long list of characteristics which he wishes to see in his dream woman. Although utterly fantastic in terms of realistic prospects, it nevertheless contrasts with Benedick's view earlier in the narrative, where he insists that he will "die a bachelor." His dogmatic disposition is obviously being broken down in small steps. The comment suggests that Benedick is not as opposed to the notion of taking a woman as he may be trying to convey; it is more a reluctance to settle for less, as it were. As we shall see, Beatrice proves to be this catalyst for his transformation.
The second extract follows the planting of the idea that Beatrice is in love with Benedick into his mind, by Don Pedro, Claudio and Leonato. Once again, Benedick launches into an excitable monologues, consisting of a few very long declaratives. This certainly displays a lot of excitement on his part, and the fact that he uses an interrogative, a rhetorical question ("love me?"), shows a disbelieving sense of glee and does much to counteract the pessimistic Benedick from a few lines earlier.
In conjunction with his earlier soliloquy, Benedick attempts to dismiss his earlier ideas, stating that "a man loves the meat in his youth, that he cannot endure in his age." He seems to think that it is acceptable that he can change his views drastically over the space of a few minutes without any sort of continual repercussions. His metaphor serves as a hyperbolic piece of irony, the imagery not befitting the very short space of time that Benedick is describing.
As Beatrice enters, we see Benedick misinterpreting her words in a very optimistic manner. He believes that her words truly confirm her feelings for him and fails to see the absurdity of his thoughts. The transformation, if you will, is complete.
Benedick even goes so far as to recite some of the qualities listed and how they are apparent in Beatrice. Again, the use of tripling is effective, but here instead to show a rush of revelationary excitement that his slim hopes have been realised.
Overall, the second extract serves to prove the insecurity conveyed in the first. Shakespeare presents Benedick as unsure and unnecessarily dogmatic, and this is confirmed by his behaviour in the second part.
In the scenes leading up to this, we see the establishment of Benedick as the witty cynic, at odds with traditional values. I believe that Shakespeare's initial presentation of Benedick goes in stark contrast to his true character. In the first scene, he is called "a good soldier." The later war of wits with Beatrice, and his misogynistic advice to Claudio and Don Pedro very much establishes Benedick as a "man's man," if such a thing existed in Elizabethan times. However, I believe that this is merely a faï¿½ade, and that he does have a pretty clear motive.
It seems to me like the reason for Benedick's disposition is, put simply, Beatrice. He is confused and frustrated at his feelings towards Beatrice, and attempts to counteract them through his words of scorn. Quite clearly, the other characters see through this, and this is what leads them to trick him later.
Another aspect of Benedick's personality, his insecurity, also seems to stem from Beatrice. At the party scene, he shows exasperation at her description of him as a "jester" and "dull as a great thaw." Benedick clearly cares about Beatrice's opinion of him, no matter what his exterior may suggest. His short monologue at the end of that scene is once again one where interrogatives are used in order for Benedick to reassure himself.
Benedick is seen to have a sharp-tongued speaking style interspersed with witty metaphors and riddles. This singles Benedick out of someone with a high level of intelligence, but it also immediately identifies him as a counterpart to Beatrice. His militant anti-marriage stance is mirrored by hers, and his words of advice to Claudio convey his emotions in flowery prose, perhaps suggesting a reluctance to disclose any true information about himself.
Overall, Benedick is presented as someone who is heavily influenced by Beatrice, and it seems that it is her actions that shape his personality, and define him as a character. Obviously, this is consistent with the narrative, and goes a long way to explain Benedick's sudden change of heart in Act 2 Scene 3.