The 48-year-old writer-producer, who replaced Russell T. Davies off screen as Who showrunner last year, told the show's official magazine that he was initially confused over the job.
"There was definitely an element of 'This is the time I'm supposed to do this...' But having said that, I wouldn't derail my whole life or anything. I'm too selfish.
"When I really cleared my head, I thought, 'I just want it. I just want it'. I couldn't quite say yes, but I knew I wasn't going to say no."
He added that taking on Who means that he is unable to pursue other projects.
"There will be a number of things I'll never, ever write, because I'm doing Doctor Who. There will be a number of things I'll never do because I'm doing Doctor Who.
"I know what they are! There will be things that won't happen. Doctor Who is like an eclipse. It blocks out the sun. It's knackering."
> Moffat: 'Who' will be stressful