Why does Shakespeare return us to Lorenzo and Jessica? What can we glean about their relationship?

One of the most ridiculous moments in this act involves Lorenzo and Jessica, who compare their love with the three disastrous love stories. They invoke Troilus and Cressida, Pyramus and Thisbe, and Dido and Aeneus as their models. This is ironic in the highest degree because all the invoked lovers are failures. For example, Pyramus and Thisbe commit suicide, and Dido kills herself when Aeneus leaves her. This hearkens back to the ease with which Jessica handed over the casket in the previous acts. Their love never underwent any form of test, either with the casket, or with the rings, which Jessica apparently trades for a monkey (3.1). Thus they in a sense condemn their love to failure like those of the failed lovers.