Why does Pip to impart as much knowledge to Joe as he can?

Because he is very fond of him and he is truly grateful, but - on the other hand - now that he is a "gentleman" - he is deeply ashamed with Joe and his "coarse" manners. But this is only in the beginning, when he would rather avoid him altogether.
In the last part of the novel, he joins in Biddy's efforts to teach Joe to read and write.