The screen legend also spoke with ET about being married four times and his deeply personal new memoir.
William Shatner is boldly opening up about his life, his loves and his eventual legacy.
The Emmy-winning TV legend's new memoir, Boldly Go: Reflections on a Life of Awe and Wonder, is a candid reflection on the actor's storied life and his journey, through his relationships with his former co-stars and his four marriages.
Shatner, 91, recently sat down with ET's Matt Cohen and opened up about the tales shared in his new book -- including his past romances and multiple marriages.
"When I got married, it was for forever," Shatner shared. "I was deeply in love. They were my best friend, and actually the marriages I've been in were for great lengths of time... so I had a full relationship."
"What happens to people, everybody, is you grow, and some lucky people who manage to be able to be aware that, they should grow together. So, like vines intertwining, you grow up the same obstacles," he continued. "A lot of people, myself included, grew apart. So at some point, you think, 'I'm not in the relationship I thought I was.'"
"So the choice is: do you stay forever or do you say, 'That's a part of my life, our life, that’s over. Let's try again.' And that's, I think, what I do," Shatner added. "Nothing is for sure, and that's what's for sure."
Apart from his romantic relationships, Shatner also dives into some of his high-profile friendships and past connections with a few of his Star Trek co-stars, including his famously tumultuous friendship with Leonard Nimoy.
"Leonard was a wonderful guy who struggled, the same way I did, with family and children and career and gradually made his way up, through talent and attendance," Shatner recalled. "We were like brothers. I loved him as family."
Unfortunately, toward the end of Nimoy's life -- before the actor died in February 2015 at the age of 83 -- the two had a falling out, which Shatner says he still doesn't fully understand.
"I don't know what happened. He wouldn't answer my calls, I wrote him a heartfelt letter saying how much I loved him and I wished him well, because he was dying. I wanted to see him, and I am hurt, and so, devastated," he shared. "Somebody said they've known people who got ill and didn't want to see anyone, because they were ill, and I'd like to say that was the reason."
Shatner also fondly remembered his former co-star Nichelle Nichols, who passed away in July at age 89.
"Nichelle was this great wonderful talent. Nobody knew just how talented Nichelle was, a singer, dancer, actress," Shatner reflected. "She was a beautiful singer and she was a lovely actress and a beautiful woman."
"And then, as time went on, she became an inspirational woman using the fiction to become fact," he added, referring to Nichols' work with NASA as part of an outreach program to inspire an interest in science and space exploration in young minds. "So she was a terrific lady."
As for the pair's historic Star Trek kiss -- the first on-screen interracial kiss in TV history -- Shatner said he didn't think about the iconic moment in those terms.
"It wasn’t anything at the time but a beautiful woman and a friend, and we kissed enthusiastically," he shared.
The final pages of the book look at what the actor's future legacy may be, and what he hopes to be remembered for, and Shatner says he really would love for his legacy to be any impact he might have on the way people treat the planet.
"I’ll tell you what my legacy is: One of the songs I performed in the Kennedy Center ... is called 'So Fragile, So Blue,' and it’s about this trip I went on into space," he said, referring to his trip on the Blue Origin mission, funded by Jeff Bezos, late last year. "The way it affected me when I came down, I was weeping. I was in grief. I was in grief for the world and the potential destruction by us. So, 'So Fragile, So Blue' is the song about that trip and my feeling and how we are killing ourselves and how we have to stop."
"Hopefully it’ll be a calling, like 'We Are the World,'" he added. "We need to be more aware than ever about what’s happening to our beloved earth. So beautiful, so connected, it's fraying at the edges. Maybe that’s my legacy."
Shatner's book, Boldly Go: Reflections on a Life of Awe and Wonder, hits shelves Oct. 4.
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