Shylock, although he is a major influence in the story, only appears in five scenes.
The most revealing of his feelings for Antonio are in Act I, scene 3. He tells us that he hates Antonio because he is a Christian and that he hates "our sacred nation".
Later in the scene, he talks about all the abuse he has suffered at Antonio's hand and Antonio makes no bones about his hatred of Shylock. It would seem that the feeling is mutual.
The pound of flesh is made in a merry jest but when Shylock has a chance to get his revenge against his enemy, he doesn't hesitate.
Business is one thing but this is also personal. The hatred is mutual.