Complex sentence: It has been suggested that Salinger himself related so closely to Holden that he was protective of the character and this was the reason he was unwilling to allow filming of the book or use of the character by other writers.
Split into simpler sentences: It has been suggested that Salinger himself related so closely to Holden that he was protective of the character. This was the reason he was unwilling to allow filming of the book or use of the character by other writers.