In The Merchant of Venice, what reason does Shylock give for wanting the pound of Antonio's flesh?

Revenge, hate, pure and simple. He says this when they offer him three times the money and he refuses it.
(Act IV Part 1 Line 60-61) "So can I give no reason, nor will I not, more than a lodged hate and a certain loathing" In other words, what reason do I need to hate him?
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