Former world champion Niki Lauda believes Michael Schumacher regretted retiring from Formula 1 racing at the end of the 2006 season.
The seven-time champion is coming back to F1 racing at this month's European Grand Prix, where he will replace the injured Felipe Massa at Ferrari.
Schumacher has been working hard to prepare for the race, as he faces the challenge of driving the F60 car for the first time on a track he has never been to.
It will not be the first time Schumacher races since he retired, however, as the German has competed in Superbike events occasionally.
Lauda reckons Schumacher still has the competition bug, and he says the most successful F1 driver regretted quitting the sport.
"He was never able to detox himself of the racing bug, as we have seen with his on-track motorcycle activities. In my opinion he retired at the end of the 2006 season because he couldn't see a real challenge - and probably he regretted his decision," Lauda told the official Formula 1 website.
"Now he's got the unique chance to step in for the injured Massa to help his former team, and to find out for himself how competitive he still is. This is something that would also interest me, because this hunger for competition - for the adrenalin rush - never dies. It's in our DNA.
"And the situation as it is now at Ferrari, with a recovering Massa and an available cockpit, gives him the chance to explore how far he's off the top. That is a question that always puzzles a top driver like he was."
Lauda, who also made a sensational comeback from retirement in the 1980s, reckons Schumacher is treating his return as an experiment.
"I actually won the second race back after my retirement, but Michael is not racing for any championship. I came back because I wanted to win, if my comeback was successful. For Michael it's nothing more than an interesting experiment."
And the Austrian doesn't believe Schumacher will have any fitness problems despite concerns about his neck.
"Forget about the age," said Lauda. "He is fit, he's undergoing rigorous training and mentally he is taking up the challenge. He will do everything possible to be fit as a fiddle in Valencia.
"True, there are some obstacles that he has to overcome, like his unfamiliarity with the track and that he's had no chance to test this year's car, but that's part of his challenge and that's why he's doing it. And that's why the whole world will be watching."