We see a particular theme central to the courtship of Portia: that theme being the deceptive nature of outward appearances.
To select a worthy groom for his daughter, Portia's father designed a test by which a suitor would have a choice between three boxes. The first, gold, promises that the said suitor "shall gain what many men desire," while the second, silver, promises that he "shall get as much as he deserves," while the final one, "dull lead," demands that one "give and hazzard all he hath."
Over the course of the play, we see the first two boxes chosen respectively by two separate suitors (the Princes of Morocco and Aragon), and, in both cases, the suitors are rebuked for the choice they selected. As the Prince of Morocco famously reads, "all that glitters is not gold."
Now, the question emerges, how does this relate to Bassanio's speech? Note, Bassanio gives this speech while making his own attempt at this test. Thus, this entire speech is shaped by the context of what has already happened in the scenes which preceded it and the themes established therein: looking beyond surface appearances in favor of the truth found beneath them. As the speech states:
There is no vice so simple but assumes
Some mark of virtue on his outward parts.
How many cowards, whose hearts are all as false
. . . wear yet upon their chins
The beards of Hercules and frowning Mars
Thus, we come to the choice of the caskets, and what I personally find particularly noteworthy is Bassanio's rejection of "gaudy gold, / Hard food for Midas" (and consider that particular reference; how the myth of Midas proceeds). As Bassanio says of lead, "thy plainless moves me more than eloquence."