Although he does not recognize it himself, what makes the stakes of the battle with the marlin so high is the notion of respect. The old man derives respect from others with displays of strength and prowess. Santiago himself feels great awe and respect for the marlin, repeatedly emphasizing this during his struggle and after he has killed the fish. Additionally, the old man’s friendships are based on mutual respect.
The old man respects the fish because the fish has all the attributes he himself lost in old age.
The old man persists in killing the marlin simply because he wants the respect of others.