What is the relationship between Shylock and Antonio in The Merchant of Venice?

The relationship between Antonio and Shylock is contentious; Antonio is heroic, but Shylock is villainous. Certainly, they are rivals in their moneylending: Antonio is kind and generous while Shylock is selfish in all aspects of his life.

One reason that Antonio and Shylock are such rivals as moneylenders is the fact that Antonio allows those who have borrowed from Shylock to come to him in their desperation when their loan and interest are due and,...

The relationship between Antonio and Shylock is contentious; Antonio is heroic, but Shylock is villainous. Certainly, they are rivals in their moneylending: Antonio is kind and generous while Shylock is selfish in all aspects of his life.

One reason that Antonio and Shylock are such rivals as moneylenders is the fact that Antonio allows those who have borrowed from Shylock to come to him in their desperation when their loan and interest are due and, in his liberality, he lends them money at the last minute. On the other hand, Shylock is completely materialistic. Not only does he demand the payment on his loans, plus his usury charge, but he is most concerned with his own personal material possessions. When, for instance, he is told that his daughter Jessica has run off with a Christian to be married, he seems more concerned about his money:

My daughter! O my ducats! O my daughter!
Fled with a Christian! O my Christian ducats! (2.8.15-16). 

Another reason that Antonio and Shylock are at ends with one another is that they play opposing roles. Shylock is the villain, the obstacle to love, while Antonio aids his friend Bassanio in romance; his love for Bassanio follows the Renaissance concept of friendship, a concept which overrides even romantic love. For, Antonio is willing to lose his life by giving up a pound of flesh so that his friend can marry Portia. But, Shylock is greedy and vindictive; he demands his pound of flesh because he hates Antonio.

More than a lodged hat and a certain loathing
I bear Antonio, that I follow thus
A losing suit against him. (4.1.60-63)

So strong is this hatred against Antonio that Shylock refuses payment, even three times the amount.