When Benedick begins to feel that he is in love with Beatrice, he also begins to feel blue because he now sees himself as the victim of his own joke. He, like, Claudio, used to mock men who fall in love and mock the institution of marriage. When Claudio fell in love with Hero, Benedick saw Claudio as a hypocrite who was now the "argument of his own scorn by falling in love," in other words Claudio became his own target for his own joke about love (II.iii.10-11). Now that Benedick feels he is in love with Beatrice, he also feels that he has become the target for his own joke. Not only that, he is worried that Don Pedro and Claudio will both laugh at him endlessly when they find out, due to his previous ridicule of both love and marriage (II.iii.214-215). Hence, when Claudio and Don Pedro see Benedick after tricking him into falling in love with Beatrice, they rightly perceive that Benedick is feeling blue. When Benedick blames his glum feelings on a toothache and they tease him further by advising him on how to get rid of the toothache, Benedick replies:
Well, every one can master a grief, but he that has it. (III.ii.25)
What Benedick means by this is that everyone loves to counsel a person out of his/her problems or griefs when they have no problems or griefs themselves. The person who finds it hardest to find a way out of his/her grief is the one who actually has the problem or grief.
Benedick's sentiment is later echoed in the play by Leonarto, making an ongoing theme of the handling of grief. After Hero is wrongfully shamed by Don Pedro and Claudio, Leonarto falls into a state of severe grief because both his own and his daughter's reputations have been ruined. When Antonio advises him to let go of his grief before he kills himself, Leonarto says to bring him a man whose daughter has been as equally loved as Hero and as equally wronged that is passing it off with a shrug of his shoulders then from that man he will "gather patience" and shrug off his grief, "But there is no such man" (V.i.9-21). Leonato further echoes Benedick's sentiment by saying that only men who don't feel grief themselves can counsel other men into letting go of their griefs, as we see in his lines:
[F]or, brother, men
Can counsel and speak comfort to that grief
Which they themselves not feel; but, tasting it,
Their counsel turns to passion. (V.i.21-24)
Hence, we see through both Benedick's line and Leonato's echoed sentiments that one of the play's themes concerns how humanity deals with its grief.