Mark Harmon's departure from NCIS came as a surprise to many fans of the show. While there has been no official word from Harmon or CBS, there are a few possible explanations for why he may have left.
One possibility is that Harmon was simply ready to move on from the role after 12 years. He has spoken in the past about how he doesn't like to stay in one place for too long, and it's possible that he felt it was time for a change.
Another possibility is that Harmon was unhappy with the direction of the show following the departures of Michael Weatherly and Pauley Perrette. Both actors were popular with fans, and their exits may have left Harmon feeling like NCIS wasn't the same show that it used to be.
Whatever the reason for his departure, Mark Harmon will be missed by fans of NCIS. He was an integral part of the show, and his absence will definitely be felt when NCIS returns for its 17th season without him.