What was Bassanio's reason for not choosing the gold and silver caskets in Shakespeare's The Merchant of Venice?

In act three, scene one, Bassanio approaches the three caskets and tries his luck winning Portia's heart by choosing the correct box. After examining the gold box, he concludes that people are often fooled by appearances and gives several examples of how appearances can trick the mind. He mentions that sinful men often conceal their transgressions behind holy appearances, while ugly women hide their blemishes using makeup. He decides that he cannot judge a book by its cover, which is why he passes on the gold box.
Bassanio uses the same logic for overlooking the gold casket as he does for the silver casket. Instead of choosing the gold or silver casket, he chooses the lead box, which is the correct casket that holds Portia's portrait inside. He chooses the lead casket because of its humble appearance, which seems to affect him more than the other two caskets. Bassanio mentions, "Thy paleness moves me more than eloquence" (Shakespeare, 3.1.108). After opening the correct lead casket to see Portia's striking image, he marries the beautiful, intelligent heiress of Belmont—before discovering that his friend is in serious trouble.