what reasons does he give for hiss decision?

His desire to run away (I'm assuming you are still talking about Launcelot here) is a mere matter of being unhappy in his servitude. Yes, he's conflicted about running away......... his father depends on him, he's honest and running away would make him otherwise, but his main reason is nothing more than he believes Shylock to be a kind of devil, mostly because he's Jewish.