How does Shakespeare make Act II, scene viii, from the Merchant of Venice such a dramatic moment?

This scene depicts a conversation between two of Antonio's friends, Salanio and Salarino - gentlemen we met in Act 1, when they were with him.

The two are discussing our protagonist, Antonio, and his enemy, Shylock. The mere mention of these two names in the same conversation already creates drama since the two despise one another so much. What makes this particular conversation even more dramatic is that Shakespeare has the two characters talk about...

This scene depicts a conversation between two of Antonio's friends, Salanio and Salarino - gentlemen we met in Act 1, when they were with him.

The two are discussing our protagonist, Antonio, and his enemy, Shylock. The mere mention of these two names in the same conversation already creates drama since the two despise one another so much. What makes this particular conversation even more dramatic is that Shakespeare has the two characters talk about Shylock's excessively exaggerated reaction to his daughter Jessica's elopement with a Christian, Lorenzo, and how this incident could affect Antonio.

Shakespeare has the two characters repeat Shylock's cries as he went about the streets of Venice searching for Jessica. The men mock him but are, at the same time, afraid of how this event will impact on Antonio, who had come to Bassanio's defense when he negated a claim that Jessica and Lorenzo were on Bassanio's ship en route to Belmont. Salanio and Salarino obviously know how spiteful and vengeful Shylock is and fears that he may want to punish Antonio for protecting Bassanio. They hope that he does not give Shylock a means to get back at him and that he has to ensure that he repays the Jew's debt.

It is, however, clear from what Salarino says that Antonio may, indeed, be in some danger, for he had been informed by a Frenchman that a richly laden Venetian ship had suffered some disaster in the English channel, losing its entire cargo. Salanio tells him to inform Antonio of this travesty in such a way that the news may not come as too great a shock.

Shakespeare reminds the audience of Antonio's vulnerable state by having Salarino recount how he witnessed him taking leave of his closest friend and confidante, Bassanio, and how heartbroken the merchant was. Added to this, Salarino also emphasizes Antonio's magnanimity when he told Bassanio not to worry about the contract he had made with Shylock for it would be taken care of.

Salanio, obviously concerned about Antonio's well-being, then suggests that the two of them should seek Antonio out and give him some cheer. At this point of the play, the audience is aware of Shylock's malice and would probably share the two gentlemen's sentiment to a greater or lesser degree.  

This scene sets the audience up for greater drama which is about to unfold since it foreshadows events later, when Antonio loses all his ships and forfeits on the bond. Shylock then has him arrested and demands recompense in the form of a pound of Antonio's flesh.