Pride is used as a central theme not in a negative way, but as a positive feeling about one's own abilities. While Santiago is very prideful, refusing to admit that he is poor and has no food to Manolin, he is also practical, and when Manolin brings him food he offers a cursory rejection and then accepts it. Later, as he fights the marlin, Santiago refuses to give in because of his pride; this is not because he is arrogant, but because he knows that this fish above all others is great enough to kill him if it is possible, and that to offer less than his best effort would be an insult. Finally, he reflects on his role as he tries to return home:
You did not kill the fish only to keep alive and to sell for food, he thought. You killed him for pride and because you are a fisherman. You loved him when he was alive and you loved him after.
(Hemingway, The Old Man and the Sea, Google Books)
Santiago is prideful because he has spent his life on the water, and knows that he is a good fisherman. While the teasing of the other fishermen does not bother him, he still wants to prove that he has the skills to catch a great fish. His pride takes him to the limits of his ability, and keeps him alive until he can show off his catch, even as a skeleton.
Hemingway shows pride as a theme in this novel through the two characters Santiago and Manolin. Even though he is not perfect, Santiago is still proud of himself and that makes him a realistic and believable character.
Perhaps pride is in this novel to illustrate an element of the human condition: that even though we are all imperfect beings, there are still some things we can take pride in.