what saddens Santiago when he thinks back to the time he caught the female marlin?

Santiago was saddened by the males refusal to leave the female....

"... the male fish had stayed by the side of the boat. Then, while the old man was clearing the lines and preparing the harpoon, [49] the male fish jumped high into the air beside the boat to see where the female was and then went down deep, his lavender wings, that were his pectoral fins, spread wide and all his wide lavender stripes showing. He was beautiful, the old man remembered, and he had stayed. That was the saddest thing I ever saw with them, the old man thought. The boy was sad too and we begged her pardon and butchered her promptly."