Delph completed an £8.5 million ($11m) move to Everton last week, ending a four-year run with the City side he joined in the summer of 2015.
Having won the Premier League twice with the Manchester club, Delph made the decision to leave in search of more regular playing time this summer, with Everton his landing spot heading into the new season as he hopes to keep his place in the England squad.
"From a personal point of view, I felt it was time for a change," he said to The Daily Mail. "I could have stayed there. They were happy with me. I wasn't told to leave. I played quite a key role within that squad.
"I had a fantastic four years there, thoroughly enjoyed it, but I wanted a new challenge. I wanted to play more football.
"Playing for England is massive for me, so if I'm playing more regularly, there's more chance of me being selected.
"I've got a great relationship with Gareth [Southgate] and the England lads, we're a really close group. The mentality there is fantastic."
Delph has been a regular for England under Southgate and started the Nations League semi-final loss to the Netherlands earlier this summer.
The move to Everton should offer a regular starting spot for the 29-year-old, though it also brings an adjustment to a new manager.
And while Delph was impressed with Pep Guardiola's style at his old club, he claims training under his new boss Marco Silva is tougher than it was with the Premier League champions.
"The volume is a lot higher, we cover a lot more distance," he said. "There's a lot more intensity and you definitely feel like you've worked come the end of the day.
"I love it. There's nothing worse for me than going home with energy.
"City's training is very good, it's shorter and sharper and is the way the manager likes the lads to play."
Having drawn 0-0 with Wigan on Tuesday, Everton will take on Werder Bremen in a pre-season friendly on Aug. 3 before facing their first Premier League test of the season against Crystal Palace a week later.