Santiago is Manolin’s mentor. Manolin idolizes him and pities him at the same time. To Manolin, Santiago is a father-figure. Father-figures and mentors are initially perceived by their admirers as invincible. Eventually, they are dethroned. This changes the relationship dramatically. Manolin still admires Santiago but he, and Santiago, must endure the other fishermen’s ridicule. Manolin’s loyalty to the old man has replaced his unwavering admiration.
When Santiago returns with the half-eaten marlin, he has succeeded and lost. This parallels Manolin’s concept of him as a fallen hero whom he is still loyal to. Manolin cries because he empathizes for the old man who had been in a slump. He finally caught a great fish only to lose it to the sharks. The other fishermen recognize the greatness of the catch, but it is still a failure because the fish is not intact. The half-eaten fish symbolizes the old man: a distorted and lesser version of its (his) former greatness.
Although Santiago reclaims his public respectability as a fisherman, Manolin cries because this success was also a defeat. It is a further dethroning of his idol. Manolin remains loyal but also recognizes that mentors and father-figures are destined to be dethroned and there is a loss of innocence and naivety that comes with this realization.