Jim Parsons, who plays Sheldon Cooper on the hit comedy The Big Bang Theory, has explained what led him to decide it was time to walk away from the show.

The beloved CBS comedy will end this week, after 12 seasons, with two episodes: The Change Constant and The Stockholm Syndrome.

It was ultimately Parsons, 46, who thought this would be the right time to bring the show to an end.

"It was the first time in my life of doing this show that it occurred to me that I might want to not do another contract after [season] 12 was up," he told The Hollywood Reporter.

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"I don't know if it's because I'm an Aries or just because maybe I'm in touch with myself. Whatever it is, once I had that thought, I was like, 'Well, that's your answer.'"

He added that there was no specific reason why he decided to leave.

"There was no situation that I was like 'well, I've had enough of that.' No. There was nothing like that. It was just…when you know, you know.

"And you're susceptible and thrown around by the whims of your own existence and getting to a certain age and your life changes and suddenly you just think different," he added.

The show creator Chuck Lorre added that he simply couldn't wrap his head around the idea of "going on without the whole ensemble" so, when Parsons decided he'd had enough, Lorre knew it was time to end the show.