The answer is stated directly by Pip in Chapter 15 of this novel, but before jumping to his reason, let us be aware that Chapters 13 to 15 deal with Pip's increasing awareness of his own humble position and station in society. Chapter 14 in particular, positioned after Miss Havisham pays the cost for Pip's indentures, focuses on how Pip's view of the world changes dramatically in the space of one year thanks to his introduction into high society and his acquaintance with Miss Havisham and Estella. Therefore, when he teaches Joe to read, he does it out of a sense of wanting to change what he sees to be his common origins. Note how Pip explains his actions in teaching Joe therefore:
Whatever I acquired, I tried to impart to Joe. This statement sounds so well, that I cannot in my conscience let it pass unexplained. I wanted to make Joe less ignorant and common, that he might be worthier of my society and less open to Estella's reproach.
What is interesting about this is the motives that Pip claims. He did not teach Joe out of a philanthropic interest in seeing his friend improve himself and giving him more opportunities in life. Actually, he educated Joe because of his own intense selfishness and to try and improve his own chances. The arrogance of the phrase "worthier of my society" speaks for itself. The introduction to high society has been something that is slowly turning Pip into a snob.
Pip is ashamed of Joe and the way he speaks and believes that education is the answer to changing him to be more sophisticated.