The world's most iconic cyclist is calling it quits. Speaking with a handful of news agencies, seven-time Tour de France winner Lance Armstrong said, "I can't say I have any regrets. It's been an excellent ride." After surviving a bout of testicular cancer that had spread to his brain and lungs, Armstrong won more Tour de France titles than anyone else in history. As an activist and founder of the Lance Armstrong Foundation, he has raised almost $400 million for cancer research. A symbol of overcoming adversity, Armstrong was also hounded by doping allegations, though he never tested positive. The most extensive investigation yet is currently underway.

So why is he leaving the sport now? Here are the most talked about theories:

He Couldn't Snag an 8th Title  In his most recent competition in January, Armstrong placed a disappointing 67th. As the Associated Press notes, the likelihood of winning an 8th title became increasingly remote. "The timing has as much to do with his growing responsibilities and family as it does with the physical limitations time has imposed," the AP writes. "He's tired... Armstrong will miss competing--let alone dominating a sport like none before him--but not the 24/7/365 training regimen that made it possible."

The Doping Investigation  The BBC suggests doping allegations may have hurried his exit:

Armstrong is currently facing a US federal investigation into allegations of doping after former team-mate Floyd Landis claimed he used banned drugs.

The investigation is examining whether Armstrong used government money to dope and win his seven Tours while riding with the now-disbanded US Postal team. It follows allegations made by Landis, who was stripped of the 2006 Tour de France title following a positive drugs test.

But Armstrong, who has never tested positive for performance-enhancing drugs, has consistently denied the allegations.

"I can't control what goes on in regards to the investigation," said the Texan. "That's why I hire people to help me with that. I try not to let it bother me and just keep rolling right along. I know what I know. 

Activism  In an exclusive interview with Politico's Mike Allen, Armstrong's "top adviser" (we assume he means publicist), said the cyclist will announce he's co-chairing a campaign for the California Cancer Research Act. The initiative aims to fund cancer research by upping the price of cigarette packs by $1 in California. Explaining the timing of his departure, his adviser said "Lance came back in to do two things: raise awareness for his foundation and cancer victims by putting the issue on the world stage, which was tremendously successful; and to win the Tour de France again, which turned out to be a lot harder than he thought. [He placed 23rd last year] In light of this, he feels the time is right to continue with his focus on the fight against cancer."

Politics  In perhaps the most far-fetched theory, the AP speculates that a "second career in politics" for Armstrong "does not seem out of the question." But Armstrong dismissed the idea outright.

"I don't think so. I get asked that question a lot. It's a job. It's probably many times a thankless job," he said. "If I were to run for any kind of office, it's impossible or very difficult to run right down the middle."

Elaborating, Armstrong said a bid in America's two-party system could alienate some of his fundraising partners.

"I would have to immediately alienate half of our constituents: 'Wait a minute, we thought this guy was a Republican. Wait a minute, we thought he was a Democrat.' I think the effect there would be a negative effect for the foundation. For now, absolutely not on my radar."