Readers are aware of Juliet's plan. They know she is planning to fake her death and run off with Romeo. Therefore, when Capulet says these lines in preparation for Juliet's wedding to Paris, readers know he is totally unaware of what is about to happen.
Furthermore, tension is increased when he talks about his happy feeling (my heart is wondrous light), which we know will drastically end in the morning when he finds his daughter has taken her life. Therefore, this speech increases the tension because we know Juliet has no intention of following her father's wishes.
