Jenson Button is adamant that the desire for a new challenge was his main reason for moving to McLaren.
The world champion's shock team switch was announced yesterday, to the dismay of the Brawn squad that took him to the 2009 title.
Button admitted that turning his back on Brawn, which he had driven for since its BAR days in 2003, was tough, but said the prospect of taking on Lewis Hamilton at McLaren was just too tempting to turn down.
"Our aim was always to win the world championship together and that's exactly what we've been able to achieve in 2009," Button told BBC Radio 5 Live.
"This is not an easy decision because I've spent so much time there and it has been a real emotional rollercoaster, but we all love new challenges, and for me this is something I need to do at this moment in my life.
"My goal since I was eight years old has been to win the world championship.
"I've done that now and I feel I need something else."
He confirmed that Brawn, which will become Mercedes GP next year, had ultimately offered him more money than he will receive from McLaren.
"I'm actually earning less than I would've been at Brawn, so it's definitely not the money, and everybody involved knows that," said Button.
He admitted that he owed the Brawn team - and Ross Brawn in particular - a great deal.
"We've had a very good relationship, especially with Ross - he's really helped me to this world championship, I couldn't have done it without him and it has been a real team effort," he said.
"I suppose the normal progression for someone who has won the world championship is to stay with that team, but for me this is exciting and that's what I need right now.
"It was very, very difficult, but things change and people move on and go to other teams if they want a fresh challenge."
Although Button's smooth driving style is seen as very different to new team-mate Lewis Hamilton's aggressive approach, Jenson has no concerns about the car potentially suiting Lewis more than him.
"All drivers have a slightly different driving style, but there's one style to drive a Formula 1 car in," Button insisted.
"I think when Lewis has a competitive car, he's very smooth - we saw that quite often in 2008.
"There are lots of things you can do with a Formula 1 car with set-up changes."
Button added that McLaren's long record of success makes him reasonably confident of fighting for another championship next year.
"I think with both teams that I was talking to you can say that there's a chance of defending your title," he said.
"Nobody really knows who is going to build the better car next season.
"History would say that Ferrari and McLaren would build a very good, competitive car, but last season Brawn came out with a stunner, a car that was beyond belief for a lot of teams.
"So you never really know, and I suppose you're always taking a gamble choosing any team.
"But I do know that there will be four or five teams at the front who will be really competitive, and I'm in one of those teams - and it's a team with real history."