We know that Shylock drops this false front when alone, and that Antonio drops it when with Bassiano and Gratiano. When Antonio is with Bassiano, we see different motives for investment. Shylock lends money almost maliciously; he seeks to have people (at least Antonio) at his mercy. Bassiano asks for money due to friendship, and Antonio stands for it. They have, therefore, different motives in all of this.
Because Shylock is a Jew and lends money charging interest, Antonio feels morally superior towards him. He uses any opportunity to show his scorn and publicly humiliate Shylock. This leads to Shylock resenting and hating Antonio even more.
Antonio for example has publicly spit on Shylock. When Shylock sees an opportunity to take revenge on Antonio he takes it. The both engage in trying to annoy the other. For example, to close the deal with Shylock, Antonio invites him dinner. Shylock refuses with the reason that he is Jewish and the food would not be kosher (Jews don't eat pork) however, it is seen as a social slight to not even come and be present.
The relationship between Antonio and Shylock stretches beyond Act I, but if we limit ourselves to that act, we see the following: Antonio needs Shylock, and, to a lesser degree, Shylock needs Antonio. There is therefore some degree of mutuality. They are also similar in that Antonio makes a point of saying he has not put all of his investments aboard one boat, but has spread the risk. Likewise, Shylock loans to many people. In...