The England international says in the Evening Chronicle: "Leaving Leeds United was more difficult for me than leaving Manchester United because of the connection I had with the club.
"The time for me to leave Manchester United had come. It wasn't because they asked me to go or because I wasn't good enough to perform at that level but because the chances that I was going to get were going to be limited.
"I am 26 now and I have been around for a few years. I wanted to go somewhere where I was starting afresh and where there was a new manager in place. There are also new staff here and new players coming in.
"The crowd here is desperate for success and players are desperate to live up to that and want to achieve as much as possible."