One reason why Benedick and Beatrice insist for so long that love is for fools is because they are both too proud to admit that they love the other. They both pride themselves on being independent, self-reliant people, and so the thought of loving, and thus being dependent upon another person, is anathema to both of them.
Beatrice also has another reason to resist and deny love. For a woman in Elizabethan England, marriage to a man (which is where love would ultimately, inevitably lead) meant accepting subservience to that man. Indeed, when a woman married a man, all of her legal rights were transferred to him, and a married woman was even considered to be the property of her husband. Elizabethan wives were also expected to exist first and foremost as wives and mothers. In other words, they were defined according to their relationships with their husbands and children. Beatrice, as an independent woman, might understandably resist this fate for herself and so claim that love is for fools.
Another reason why Beatrice might consider love foolish is because she seems to have a history with Benedick, the object of her love. In act two, scene one, in reply to Don Pedro teasing her about losing Benedick's heart, Beatrice responds by saying:
Indeed, my lord, he lent it me awhile; and I gave him use for it, a double heart for his single one: marry, once before he won it of me with false dice, therefore your grace may well say I have lost it.
The implication of this quotation is that Beatrice once before had a loving relationship with Benedick, but was hurt by Benedick. She says that he "once before won [her heart] . . . with false dice," which suggests that Benedick was unfaithful to or dishonest with her in some way. Beatrice may, therefore, be wary of falling in love again because she has already been hurt when in love before.