The Big Bang Theory has been one of the longest running shows on television. It ran for 12 seasons and earned a ton of Emmy Awards. The show was complimented by its terrific cast, which included Jim Parsons as Sheldon Cooper.

Parsons' performance in the series was a big part of why audiences gravitated toward the show. It made it difficult for series creator Chuck Lorre to imagine the series continuing without him. When Parsons decided to leave the series, many fans were questioning his reasons.

Parsons was no doubt a special part of the show's success. Lorre once reflected on a monologue Parsons gave in the pilot, saying it convinced him "something remarkable was happening.

"

Jim Parsons Revealed Why He Decided To Quit 'The Big Bang Theory' After 12 Seasons

After playing the role of Sheldon Cooper for 12 seasons, Jim Parsons decided to walk away from The Big Bang Theory. His decision blindsided many cast members and fans of the show, all of whom questioned Parsons' reasons.

In an interview with The Hollywood Reporter, Parsons explained why he decided to leave the series, claiming it had become "more difficult."

"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 12 was up," Parsons said.

"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 continued, "There was no factor; 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.

"

"It has been fascinating to think about who I was 12 years ago," Parsons said. "And sometimes when I have trouble learning a line or saying a line of Sheldon's right now, it's hard to know why specifically. But it's like, you're not the same person you were."

"There is a possibility that this actually became more difficult for you in a way. And I don't know what that means but it's like you just change."

Jim Parsons Fought To Keep Mayim Bialik As Part Of 'The Big Bang Theory' Cast

Jessica Radloff's book, "The Big Bang Theory: The Definitive, Inside Story of the Epic Hit Series" contains a lot of inside scoop on the show.

Much of the revelations come from the cast members themselves.

Among these revelations is the fact that Mayim Bialik's character, Amy Farrah Fowler, was originally supposed to appear in just one episode. According to the book, Parsons was extremely taken with the dynamic between Sheldon and Amy. He decided that he would fight to keep Bialik on the show if the writers ever tried to write her character off.

"They decided after that summer break that I would be brought back, but they book you one episode at a time, which is not stressful at all!" Bialik said. "After my episode in the season three finale, I literally thought my character might never come back.

I had been out of the industry for so long . . . it really could have gone either way."

She continued, "There was some negative attention when I joined the cast because some people held Sheldon very near and dear to them and didn't want him to change or become a cheesy boyfriend. To me, there was never a chance of that, and I think our writers actually handled that arc of him becoming more interested in romance very, very well."

Despite what viewers may have thought, Parsons believed his character needed to grow and evolve.

"There was no choice but to evolve the character," he said. "And at some point in season four—I don't know if something caused it or not—I remember saying to Todd, 'I will not let this character go without a fight.'"

He continued, "That was notable for me, only in that I almost never disagreed with the writers. But at some point I felt a certain way about working with Mayim that I was like, If for whatever reason we seemed to be weaning her off of this show as a character, I would go and talk to them."

Chuck Lorre Said A Monologue By Jim Parsons Convinced Him 'Something Remarkable Was Happening'

To celebrate the show's 15th anniversary, creators Bill Prady and Chuck Lorre spoke with Entertainment Weekly. The two discussed many aspects of the show's history and revealed their favorite moments in the series.

"Jim Parsons' speech about why he sits in that particular spot on the couch in the pilot episode," Lorre said when asked his favorite moment. "I think we all watched that and understood something remarkable was happening. There were no real jokes in the speech, it was just a worldview that was so precise, and he brought it to life. It was an astounding moment I'll never forget."

"I love the episode where Sheldon doesn't like that Leonard is dating Raj's sister," Prady said. "She was a lawyer and kept finding loopholes in the roommate agreement and was using it against him. Sheldon gets so mad that he ends up threatening to tell her parents she was dating Leonard and uses the Star Trek self-destruct sequence to do it."

He continued, "To me, that was this great nerd explosion. Then there's the incredible moment with the Christmas gift where Penny gives Sheldon a napkin with Leonard Nimoy's DNA on it."

Lorre added, "That's when he hugged Penny. It was the first time he laid a hand on her. That was a big character breakthrough moment, he was so alienated up to that point."