The former England captain spoke to the media on Monday after agreeing a three-and-a-half year deal with the MLS club.
Rooney only returned to his boyhood club last summer but was told he did not feature as a key player in new boss Marco Silva's plans.
"I was grateful and delighted to go back there last summer. But I've made the decision to come here and I am delighted to be here."
Rooney added: "I'm not here to see out the last few years of my career, I'm here to compete, I'm here to win and that's the way I've always played and will always play.
"Even on the training pitch yesterday, there were a few of the academy players and I had a go at them for not running as hard as they should have. That's on my first day.
"I've always set high targets. I demand a lot of myself and expect that from team-mates as well. I have to adapt to however the club is run, and embrace it."