"I'm very excited," Button told Radio 5 Live. "The season just gone has been a very special one. Since I've been with Brawn, our aim was always to win the title, and it's exactly what we did, so it wasn't an easy decision to make to leave.
"I spent so long there through the BAR and Honda years, and it's been a real emotional rollercoaster since then. It's just something I needed to do at this point in my life.
"It was 100 per cent my choice. For me, the move is because it's going to be a huge challenge to go up against Lewis in his environment. My goal since I was eight years old has been to win the world title, and I've done that now. Now I feel I need a new challenge."
The Briton denied the decision had been motivated by financial issues.
"No. I'm earning less than I would have at Brawn. I didn't move for money and everyone knows it. It's something new for me. I'll have to work very hard in the new situation I've put myself in, and I intend to do it.
"Of course it's not going to be easy, but I'm very excited about the challenge. I've got to work very hard to achieve what I want to achieve."
Button also admitted Brawn had done everything possible to try and retain him.
"Yes they did. We had a very good relationship, especially with Ross. I couldn't have done it without him. It's been a real team effort. For most people the normal progression is to stay with the team. For me though, this is exciting. It's what I need right now."
The world champion, who had been with the team since the 2003 season, is expecting his relationship with Brawn to change because of his move.
"It's going to be a different relationship, of course, because I'm not in the team any more. We're not fighting for the title any more. We've achieved that. Instead I'm going to be a rival of Mercedes Grand Prix. We'll still have a good relationship though. We've been through a lot together. "
He conceded the emotional side played a role in making his decision to leave Brawn harder, but Button said the new challenge was too exciting to turn it down.
"I was excited, but there was a lot to think about because I'd been with Brawn for seven years. On an emotional side there was definitely lots to think about.
"The challenge of going up against Lewis in his environment was a challenge. Tough, but a challenge. For me, I achieved everything I wanted to with Brawn with winning the title in '09 and I'm looking for new challenges. At this point in my career it's an exciting opportunity that I'm going to take."
Button confirmed at McLaren
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