Shakespeare frequently refers to the conflict of Reason vs. Emotion (or Judgment vs. Passion) in his plays. The religious position regarding this conflict is that Reason is the king over the emotions. Reason is a gift from God that allows all men to control their emotions. Thus, not controlling one's emotions is just a sign of weakness. Shakespeare, however, knew better. He understood that there are occasions in everyone's life when the emotions or passions are so strong that they overpower the reason of even the best and smartest men. Love is one such powerful emotion that can overwhelm the senses; but other emotions, like jealousy or anger, can also cause someone to become totally irrational and perhaps even a bit mad.
At the beginning of Act V, Antonio attempts to persuade his brother Leonato to calm down. Antonio fears that Leonato's intense emotions will bring harm to himself. But Leonato's grief and anger are so intense that he is unable to calm down. He explains to his brother that it is easy to advise or counsel patience and calmness when one is not experiencing the emotions for himself:
Men Can counsel and speak comfort to that grief Which they themselves not feel, but tasting it Their counsel turns to passion. (20-23)
By the words "tasting it" Leonato means experiencing the emotion for oneself. Antonio can counsel Leonato to be calm and cool and rational because Antonio does not feel those emotions himself. But when Antonio becomes overly emotional about some matter or another, his advice to remain calm and cool would be forgotten. He would become just as wild and irrational as Leonato is at that time.
Leonato, here, is speaking for Shakespeare himself. The playwright repeated this same idea in several other works. For example, similar words also appear in the tragedy of Othello. When the nobleman Brabanzio becomes irrationally angry because his daughter Desdemona has married Othello, the Duke of Venice advises him to accept his misfortune. But Brabanzio (like Leonato) tells the Duke that sorrow and grief "must of poor patience borrow" (Act I, Scene 3, Line 214 of Othello). Sorrow and grief are powerful emotions that patience (an aspect of reason) cannot control. Like Leonato, Brabanzio does not calm down or control his emotions.