Why do you think Fletcher insisted that Jonathan should not leave in Jonathan Livingston Seagull?

To be able to answer this question, let's review a little background of the story of Jonathan Livingston Seagull by Richard Bach. Jonathan is bored with normal seagull life and thrilled with flying. After being expelled from the flock, two gulls appear and take him to a higher plain of existence. There, he is instructed by a wise teacher named Chiang who teaches him to fly without limitations and also of the importance of love. Rather than be content with what he has learned, though, Jonathan wants to share it with those he has left behind on Earth.

And the more Jonathan practiced his kindness lessons, and the more he worked to know the nature of love, the more he wanted to go back to Earth. For in spite of his lonely past, Jonathan Seagull was born to be an instructor, and his own way of demonstrating love was to give something of the truth that he had seen to a gull who asked only a chance to see truth for himself.

Fletcher Gull is one of the young gulls that Jonathan finds is open to instruction when he returns. At first Fletcher is impatient, insecure, and angry, but he gradually becomes more skilled and patient until he is able to also become an instructor. Jonathan decides that Fletcher is ready to take over as leader while he goes on to teach other flocks.
Fletcher insists that Jonathan should not leave because Fletcher considers himself to be just a "plain seagull" and Jonathan to be the instructor. However, Jonathan points out that Fletcher doesn't need him any longer.

You need to keep finding yourself, a little more each day, that real, unlimited Fletcher Seagull. He's your instructor.