Shylock tells Antonio, "suff'rance is the badge of all our tribe". In this case by tribe, he means the Jewish religion which is important because Antonio is from a different religion which affects the way he does business and the interest he himself charges.
Shylock gets verbally abused, hustled and bullied by many characters throughout the play because even other sharp businessmen see him as being unreasonable and usurous. He gets called names including "villain with a smiling cheek, bloody creditor, and even 'damned inexecrable dog’ whilst Antonio's business practices are seen as acceptable and Shylock does not like the competition.
As he is obviously a shrewd businessman he is very observant and makes it his business to know other businessmen's news - he has found out everything to do with Antonio's business plans, and that because he is a Christian he is becoming popular because of the fair way he is perceived to conduct his business affairs - usury or 'charging interest' was frowned upon in many rreligious circles, including Christian ones.
So the top reason he dislikes Antonio is financial: "I hate him for he is a Christian; / But more, for that in low simplicity / He lends out money gratis". This means that he suspects Antonio of lending out money at much cheaper rates than he does because he is a Christian.