Benedick's first soliloquoy consists of two main parts -- the humorous ridicule of Claudio's change from soldier to lover, and the explanation of how he, Benedick, is immune to the blandishments of love. Benedick has, thus far, never found a woman to move him to love:
One woman is fair, yet I am well; another is wise, yet I am well; another virtuous, yet I am well; but till all graces be in one woman, one woman shall not come in my grace. Rich she shall be, that's certain; wise, or I'll none; virtuous, or I'll never cheapen her; fair, or I'll never look on her; mild, or come not near me; noble, or not I for an angel; of good discourse, an excellent musician, and her hair shall be of what colour it please God.
At this point in the scene Benedick thinks of love (and women) only in terms of the general. He thinks of women in terms of desired attributes only. But when faced with the prospect of Beatrice having affections for him, he changes dramatically.
They say the lady is fair—'tis a truth, I can bear them witness; and virtuous—'tis so, I cannot reprove it; and wise, but for loving me—by my troth, it is no addition to her wit, nor no great argument of her folly, for I will be horribly in love with her.
Two things occur here -- he realizes others criticize his lack of love, and also that a real, specific woman (he thinks) loves him. This makes love specific rather than general, and therefore more obtainable to him.