SARAH: Hamlet's words are certainly confusing. First he claims that he did love her. Then, when she admits that he made it seem that way, he replies that she should not have believed him. But if he did lie earlier, then why should she believe him now?
RALPH: Yes, that's not easy to understand. Perhaps Hamlet's saying that she shouldn't have believed him — in other words, shouldn't have fallen in love with him - because she should have known it would turn out badly.
SARAH: But he gives quite a different reason, saying that she shouldn't have believed him because he is not completely virtuous.
RALPH: That's right, Sarah, and he uses an agricultural metaphor to communicate this. Hamlet says that virtue can't inoculate, or be grafted onto, our original stock — when you graft, you take a small branch of one tree and cause it to grow and bear fruit on a different tree.
SARAH: He means that virtue can't completely change our old way of being — and here, our old stock refers to our original sinful nature in the Christian tradition, after the fall into sin in the Garden of Eden.
RALPH: And when he says "relish of it," he means we'll still taste the sin — even though virtue can be imposed on us, this original sin will still show through, like a bitter fruit.
SARAH: And then Hamlet changes his tune entirely, claiming that he never loved Ophelia. You see? The poor girl!
RALPH: If we follow Hamlet's lines about virtue being unable to change sin, he might mean here that the love he felt for Ophelia could not have been truly love, or virtuous love anyway.
SARAH: And she says she's "all the more deceived" — deceived here means not just misled or tricked, as it does in contemporary English — it also means disappointed, or let down. So when Ophelia says she's even more deceived in hearing that Hamlet never loved her, she's telling us she's already been disappointed — perhaps she's already realized this relationship was doomed, and now it's worse, as he's telling her that he never loved her.
RALPH: There's also maybe some irony here with the double sense of being deceived — if Hamlet is purposefully using Ophelia and the drama of their relationship to confuse Claudius and throw him off the track, then Ophelia's also being tricked by Hamlet. And of course, this whole conversation is on some level a deception, engineered by Polonius.