The Prince of Morocco is a very arrogant man. He thinks he is above the lead casket: "Hazard for lead?...A golden mind stoops not to shows of dross" (2.7.21-24). Nor will he choose the silver as it is inscribed with: Who chooseth me shall get as much as he deserves" (2.7.27-28). The Prince thinks he deserves much more!
He chooses the gold as he believes that gold and the Lady, Portia, are what we should possess and what he deserves but he is fooled by his own arrogance as All that glisters is not gold!
so that he could be the one to married her and find the casket with the poetry in it