What are the reasons which led Bassanio to reject the gold and the silver caskets and to choose the lead?

Bassanio believes that people are far too concerned what's on the outside rather than the inside. He refuses the gold casket because in his own words, "all that glitters is not gold." The silver is rejected because he sees it as the "common drudge 'tween man and man." In the end, he chooses the lead casket because the "true worth" lies on the inside, and let's face it..... who would choose lead or believe that the treasure lie inside of something so simple and plain?