"I will always have good memories of Everton but I have to admit that it had reached the stage where I wasn't enjoying my football there," Johnson told The Guardian. "Moving back south was a factor and always something I wanted to do, but the biggest factor was being happy - and the truth was I didn't feel happy during the end of my time at Everton.
"The problem came down to wanting to play in my best position and the fact I was either out of the team or stuck on the right wing, in a position I didn't like.
"I was stuck out on the right wing for nine or 10 games and I just think, over time, I lost a bit of confidence and, with that, my form. That's when I stopped enjoying my football."
Johnson has dismissed the notion that heading to Fulham is a backward step in his career.
He added: "I've heard people saying, 'He's gone from Everton to Fulham, that's a backward step, blah, blah, blah'. But I don't see it like that. I see Fulham as a massive club, one that can achieve great things."