When confronted with the fact that both Beatrice and Benedick were led, via the "noting" of planned conversations among their friends, to believe that each was in love with the other, they felt embarrassed at being tricked into admissions of love. Both are proud characters and neither wanted to be the one to say "I love you" first out of a fear of rejection. When each is asked about loving the other and both respond, "No more than reason", they mean "no more than any one human being should give a hoot about any other human being". This demonstrates their pride in not being willing to admit their true feelings. However, both Claudio and Hero produce written confessions of love that they have confiscated, and Beatrice and Benedick are forced to admit the truth.
they talk about loving to reason because they claim that they dont love eachother with a passion and until they are proven wrong they are more than happy to deny their love for eachother