Why does Bassanio come to Antonio for help?

Bassanio and Antonio are close friends. Bassanio declares that already he "owe[s] [Antonio] much," as they have known each other since childhood, but he prevails upon Antonio "to shoot another arrow that self way / Which you did shoot the first"—that is, to take a chance on Bassanio again. Antonio says that, because he and Bassanio are such close friends, Bassanio should know he is entitled to any help Antonio can give him.
Bassanio explains that he needs money to woo Portia, because he does not have "the means / to hold a rival place" with Portia's other wealthier suitors. He hopes that Antonio will help him as he has before, and indeed, although all of Antonio's fortunes are "at sea"—that is, tied up in his investments—he has good credit, and he says he will "try what my credit can in Venice do" in order to help his friend. Antonio is willing to use his own good standing to acquire a loan to help Bassanio, who does not have Antonio's business standing or personal wealth.