Why does Shylock deserve our sympathy in Merchant of Venice?

The most obvious source of sympathy for Shylock is that he is part of a despised minority, treated with contempt and loathing on the basis of his religion. As a Jew in an overwhelmingly Christian city, Shylock is subjected to all kinds of humiliating restrictions, both legal and social. Money-lending, or usury, to give it its old-fashioned name, was one of the few occupations open to Jews at that time, as it was regarded as sinful for Christians to lend money at interest. Despite having little choice in the matter, Jews were loathed by Christians for engaging in the very occupation which they themselves had forced the Jews to pursue.
Shylock's treatment by his daughter Jessica is also appalling. She runs off with her Christian lover, Lorenzo, but not before stealing some of her father's hard-earned money. Although Shakespeare's portrayal of Jessica is largely sympathetic, her behavior does come across as willful, selfish, and ungrateful. And even if we do find Shylock a tad controlling in relation to his daughter, there can be no excuse for stealing under any circumstances.
Finally, one would have to sympathize with Shylock over his forced conversion to Christianity. This comes at the end of a very public humiliation when the law is twisted and distorted to ensure that Shylock comes off worst in his agreement with Antonio. Again, whatever we may feel about the precise details of Antonio's debt to Shylock, with its notorious stipulation of a pound of flesh being forfeit, there's no doubt that forcing someone to convert to another religion is completely unacceptable.