The reason he actually gives in demanding it is "to bait fish withal". Shylock wants to use Antonio's flesh as fish bait--though one rather hopes we are to take him with some irony! But who knows?
Some commentators have recently argued that Shylock intends the pound of flesh as an absurd, comedic stake - which he will never have to claim, as it's hugely unlikely that the ships will fail to arrive--and that Shylock is trying to ingratiate himself with the Christians. It is only after Jessica's elopement that Shylock turns vengeful.
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?
Tell me if this was sufficient