In The Merchant of Venice, there is much debate over the match between Portia and Bassanio and how sincere their feelings might be. It is speculated that Bassanio only wants to marry Portia for her money so that he can secure a future for himself as he has expensive habits, having spent all his money and squandered his own wealth to the point of having to borrow from his best friend, Antonio. Then there is the question of whether Portia simply prefers Bassanio over any of her other potential suitors rather than actually feeling any real love for him. Portia feels dis-empowered by the terms of her late father's will which insist that any suitor choose from three caskets to determine which one holds her picture. An incorrect choice has serious consequences for the failed suitor. Portia is therefore anxious to secure herself a husband whom she finds most suitable.
Nerissa reminds Portia of Bassanio and Portia agrees that he is "worthy of thy (Nerissa's) praise" (I.ii.108). When he arrives, Portia tries to delay his choice claiming that she would first like his company. She offers to teach him over the course of "some month or two" (III.ii.9) but is conflicted because that would break her "oath" to her father, but if he chooses poorly she tells him that she will wish she had in fact broken her oath. She is so concerned that he may make a hasty choice and questions his motives, referring to "treason... mingled with your love" (III.ii.27).
To ensure that she has more control over the situation without directly revealing her cunning, she cleverly plays music. This music is wisely chosen as it provides a hint as to which casket would be the correct choice. The first two lines of the song rhyme with lead and her picture is in the lead casket. Therefore, it is possible that she "traps" him into thinking that he is making his own considered choice but she is actually guiding that choice.