In The Merchant of Venice, Act III Scene i, why does Tubal talk about Antonio while he is talking about Jessica?
He first talks about Jessica which makes Shylock felt very sad/angry, then he made Shylock happy by telling him about Antonio's ship. What was his motive?
Actually, in Act III, scene i, it is Shylock who is questioning Tubal about both his daughter and Antonio. The conversation between them switches back and forth almost line for line between distressing news of Jessica and how much money of Shylock's she has spent and "good" news of Antonio's failure.
Shakespeare has certainly devised that this conversation would see-saw back and forth like this from one emotional extreme to another. The best answer as to why is that this play is a comedy, and these abrupt switches provide a comic opportunity.
Shylock is a comic villain and is not meant to earn the audience's sympathy. In this scene, the audience is set up to laugh at Shylock's greedy, shallow behaviour, as he frets over the money and stuff (not his daughter) he has lost, and then gloats over the demise of Antonio, which he believes will give him power over Antonio in the fulfilling of his bond.
It is worth mentioning that the famous "Hath not a Jew eyes?" speech comes just before this interaction in Act III, scene i. Shakespeare is a master at turning audience expectation on its head, and so he sandwiches this moving speech by Shylock between a scene where he is the butt of two Venetian Christians' jokes and a scene in which his behaviour is simply greedy and vindictive.
Tubal's motive, like so much else in the play, is a matter of conjecture. One answer is that he would like Shylock and Antonio to settle their differences. This may also be the reason the Duke allows Shylock to present his case in open court. In Professor Halio's notes(Oxford editon) we find "Leah's only mention occurs here, precisely where Shylock contemplates revenge against Antonio." Therefore, one may infer that Antonio and Shylock were rival lovers of Leah, which in turn may explain, in part, Antonio's absurd comment, "The Duke cannot deny the course of law, / For the commodity that strangers have / With us in Venice, if it be denied, / Will much impeach the justice of the state,/ Since that the trade and profit of the city / Consisteth of all nations"(3.3), which is his reply to Solanio's "I am sure the Duke / Will never grant this forfeiture to hold."