The reason Santiago goes after the fish is for memories. Santiago wants to feel young again, he does this by fishing. While chasing this fish he remembers all the times he had caught giant fish. Once he catches the fish he gets attacked by sharks. He remembers when he was younger and could club sharks to death, he is only fishing to feel younger, Santiago is reliving his "glory days" when he caught many large fish. Know in his old age he hasn't caught a fish in 84 days and once he finds a large fish it is slipping through his fingers. Santiago talks about how he could catch giant fish alone and kill sharks with clubs but now he feels useless because he couldn't save his fish from the shark. He wishes that he could relive his memories and catch the biggest fish he would ever catch. Therefore Santiago goes after the fish for memories so he could feel strong and confident about himself.