Why does Santiago like the flying fish?

He hears the leaps and whirs of the flying fish, which he considers to be his friends, and thinks with sympathy of the small, frail birds that try to catch them.

The flying fish signal the arrival of dolphins, while, in Santiago’s experience, the magnificent tug on the line can mean only one thing: a marlin—a type of large game fish that weighs hundreds of pounds.

THe point him towards the deeper water and greater fish. "He hears the leaps and whirs of the flying fish, which he considers to be his friends, and thinks with sympathy of the small, frail birds that try to catch them."

Santiago likes the flying fish because they tell him when there are dolphins or other large fish nearby. This is great for Santiago, who doesn't catch fish very easily when he is going through one of those time periods. The flying fish give him hope that he can catch something huge.