What binds Antonio and Bassanio's friendship in The Merchant of Venice?

In Shakespeare's The Merchant of Venice, Antonio and Bassanio seem to have opposite personalities: Bassanio appears to suffer from a dire lack of finances, while Antonio is a successful merchant. Be that as it may, the two men are close friends, and their friendship is bound by a mutual concern for each other's well-being. 
Consider, for instance, that Antonio readily and selflessly helps Bassanio secure funds so he can court Portia. Likewise, once he catches wind of Antonio's financial difficulties, Bassanio immediately departs Belmont and sets out for Venice, despite his upcoming wedding to plan/ attend. In short, it seems both characters are ready to help each other at the drop of a hat, and will do whatever they must to ensure the other is supported. As such, it's apparent Bassanio and Antonio's friendship is bound by a strong mutual concern for each other's well-being, and this quality enables two seemingly opposite men to enjoy such a vibrant friendship.