Why does Shakespeare introduce some of the main characters as being in a sad mood? What effect does this have on the audience?

In Act one, Scene 1, Antonio is introduced as a character who is suffering from severe depression. Antonio laments about his mood and mentions that he cannot sleep or think clearly. Antonio tells his two friends, Salerio and Solanio, that he cannot figure out the cause of his bad mood. When the audience first meets Portia in the second scene of the play, she laments to Nerissa about her recent suitors. Portia tells Nerissa that she is tired of living and feels helpless in her journey to find love. Portia is portrayed as a hopeless romantic who is also struggling with depression. Shakespeare chose to introduce these two main characters who are in depressed moods to create sympathy for them. The audience has empathy for Antonio and Portia and immediately feels sorry for them. The two characters moods change as the play progresses, and the audience is relieved when Portia finds love and Antonio's fortune is saved. By portraying the two main characters as helpless at the beginning of the play, Shakespeare manipulates the audience into feelings sorry for them, then rewards the audience by illustrating how Portia and Antonio both end up happy at the end of the play.