The singer - who joined the panel with Britney Spears for this season - told Teen Vogue that the job came at just the right time in her life.
"It was a no-brainer!" she said. "Right before I got the offer, I'd been sitting in a meeting feeling really discouraged [about my career], wishing I was further along.
"And I decided that I needed to stop thinking about what I wished I had and start thinking about what I was going to have. I was ready to do something new, and I wanted to put that out there, in the universe... and then [Simon Cowell] called, literally, a week and a half later."
Lovato, who described Spears as "really sweet and funny and quirky", also revealed that she wants to help the contestants deal with their newfound fame.
"There's so much more to it than just being a talented singer," she said. "I'm going to help them mentally prepare for everything that's coming - like, all these things that have happened to me recently, I really attribute them, in part, to the work that I've been doing on myself.
"The second that I became more centred and aware of who I am, my performances got better, and I was better able to relate to people. For a long time, I was pretending to be someone that I wasn't, and I was miserable. When I came out and said, 'I'm human, I've got problems, but I'm willing to share them', that's when I started to really connect."
Lovato recently joked about Cowell on Jimmy Kimmel Live, quipping that he "shows more cleavage" than she does.
The X Factor USA airs Wednesdays and Thursdays on Fox, and on ITV2 in the UK.