Why did Schumi retire?

A question that's been burning inside me for a long time - Did Michael Schumacher retire or was he forced to quit? The actual answer may never be known. But nevertheless, it's a worthy discussion.
Ferrari claims that Schumi's decision was his own. But, the fact that they signed Kimi early this season raises some eyebrows. Did they force Schumi into deliberation by signing up his arch rival for 2007? May be or may be not.
Raikkonen is a wonderful driver. But bringing him in to partner Schumi obviously meant competition within the team. Obviously, a driver who has enjoyed the "chief driver" position in the team for this long, wouldn't like that. And it also puts a pressure on the team because, they cannot supply the best of cars to both the drivers. Anyone who has been following F1 would know this. Ferrari said that Schumacher was involved in the decision to sign up Kimi. Why would the Ferrari ace want to threaten his position in the team, or put pressure on the team which he resurrected? I do not understand.
Also there was the rumour of the Moto GP champion Valentino Rossi joining Ferrari to try his luck in F1. This was confirmed when Rossi expressed his interest in public. The dream team of Schumi-Rossi was in every fan's lips. But something happened and everything came to a lull. What actually happened still remains a mystery, even though Rossi revealed later that he did not want to experiment Formula One. His initial interest and then this back-out arouses a lot of suspicion.
Regardless, Schumacher wanted to retire a eight-time champion. He did ask to divulge his decision after the season ended. This hinted that he wanted to retire or continue based on the champioship result. But Ferrari declined his request. At San Marino, he had no choice but to make an announcement. And he chose the snowy peaks of Swiss to the roaring engines at the Ferrari pits.
Also, there have been a number of people leaving the Ferrari team. Technical Director Ross Braun is leaving the Italian stables (he's supposedly under sabbatical) and engine guru Paolo Martinelli is leaving Ferrari to join the parent company Fiat. And Jean Todt has been promoted as CEO. Something fishy is going on for sure inside the Italian team. And just now I read an article which quotes the return of Piero Lardi Ferrari, the illegitimate son of Enzo Ferrari in the team.

"Lardi was also responsible for the Ferrari F1 Team prior to the arrival of Michael Schumacher. Whereas today's Ferrari Team is a model of precision and efficiency, the pre-Schumacher Ferrari Team was riddled with political intrigues and infighting. Ferrari pit stops from that were more likely to be position losers, than race winners.
When Schumacher joined Ferrari in 1996, critics suggested that it was a career-ending move! The Ferrari of the day was erratic, occasionally fast, but not a serious contender. It took Schumacher until 2000 to win his first Championship with Ferrari, and that Championship broke a twenty-one year losing streak at Ferrari. At the helm during many of those losing years was Piero Lardi Ferrari."

Well, could this be the reason for Schumi's retirement?