Some examples of the lead actor deciding that they didn’t want to do the show anymore:
*Ted Danson (https://www.quora.com/Which-actor-or-actress-contributed-to-the-cancellation-of-a-TV-series/answer/Amy-Christa-Ernano) - Cheers

*Chuck Conners (https://www.quora.com/Which-actor-or-actress-contributed-to-the-cancellation-of-a-TV-series/answer/Mario-Martini-6) - The Rifleman

*Redd Foxx (https://www.quora.com/Which-actor-or-actress-contributed-to-the-cancellation-of-a-TV-series/answer/Darrell-Blank) - Sanford & Son

*Andy Griffith - The Andy Griffith Show

*Will Smith - The Fresh Prince of Bel-Air

*Bea Arthur - Maude and The Golden Girls

Examples of actors whose untimely deaths in all likelihood contributed to their shows suddenly ending:
*Jon-Erik Hexum (https://www.quora.com/Which-actor-or-actress-contributed-to-the-cancellation-of-a-TV-series/answer/Michael-W-601) - Cover-Up

*Phil Hartman - Newsradio

*John Ritter - 8 Simple Rules

*Ted Knight - The Ted Knight Show (later retroactively becoming the sixth season of Too Close for Comfort)

*Freddie Prinze - Chico and the Man

Tim Allen and Patricia Richardson- Home Improvement

Sarah Michelle Gellar - Buffy; SMG wanted to end things.

Nancy McKeon and Mindy Cohn - The Facts of Life stars were ready to move on; Mindy even phoned in her final episode.

The cast of Friends - most were ready to move on, and as a group, they refused to do more seasons.

According to the late Charlotte Rae, 'Car 54, Where Are You' star Joe E. Ross's antics and 'tude became so obnoxious and overwhelming to all that the showrunner Nat Hiken pulled the plug on the entire show!

Dan Blocker's death, as well as moving the show to Tuesday night, undoubtedly hastened the demise of Bonanza.

According to the late Charlotte Rae, 'Car 54, Where Are You' star Joe E. Ross's antics and 'tude became so obnoxious and overwhelming to all that the showrunner Nat Hiken pulled the plug on the entire show!

That reminds me that I had previously started a thread (https://www.sitcomsonline.com/boards/showthread.php?t=372310) about shows that were cancelled primarily because of the stars' misbehavior.