Each of these characters comes to the bond with very different intentions. In the bond, we see in one sense the power of the market to bring people of different religions together despite the sectarian divisions inherent in their society. In the bond they agree upon, we see that each party is ready to risk something.
In Antonio's case, he is willing to risk his life. The terms upon which the bond is drawn up require Antonio to forfeit an amount of flesh whose weight equals in value the gold he failed to pay Shylock (1.3.145). If his ships do not make it safely to port, Antonio will be in danger of death from loss of blood. Antonio takes this risk at a potentially enormous cost for no material profit. What the generous merchant forgoes by way of material wealth, he makes up in the loving loyalty of fraternal friendship.
In contrast, Shylock makes the bond for motives of vengeful spite. Before making the bond, he catalogs a number of abuses meted out by Antonio (1.3.106). He brings these abuses up to shame Antonio as a hypocrite.
In summary, Antonio reveals himself to be a charitable, altruistic individual, while Shylock reveals himself to be unforgiving and argumentative.