The Europa League winners confirmed the capture of the 20-year-old midfielder on a four-year contract on Tuesday, and he undertook his first duties as an Atletico player by speaking to the press at the Vicente Calderon today.
"There are circumstances in life," he said. "I was very comfortable at Arsenal, but with the interest of Atletico I did not hesitate.
"I talked to my agent and I told him to deal with such an offer. "
Merida had been at Arsenal since 2006, having previously been part of the youth set-up at home-town club Barcelona, but his deal with the Gunners was due to expire on June 30.
"Wenger looked after me and my family since I arrived in London," Merida continued. "I have a very good relationship with him.
"I am and I will be eternally grateful.
"Obviously he is not pleased with my departure, but he wished me luck and behaved like a gentleman."
Merida expects his game to be ideally suited to the Primera Division.
He added on the club's website: "The English league is more physical - very strong players with a very direct game. The Spanish league has a lot of quality and will benefit my game.
"I am proud to wear the red and white shirt. I will give everything for the team."