Downing admits manager Sam Allardyce had a big say in his decision.
"There are good ambitions here - moving to the Olympic Stadium and with Andy signing, and with the manager wanting to keep strengthening," Downing told the Telegraph.
"I was happy to make that move. It's a great club, a club on the up and finished 10th last year and we want to finish as high as we can.
"He's made it clear to us that he wants to finish higher and I think we can do that. I liked his [Allardyce's] ambitions.
"He's not one to sit around, Sam, he wants to keep improving so I thought it was a good time to come here.
"He's quite persuasive and he's very straightforward, which is all you can ask for as a player.
"He told me to come here, come and enjoy my football and he would improve me as a player - simple things but things that you want to hear."
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