Spies announced via his own Twitter feed last July that he was quitting Yamaha's factory MotoGP team.
After a period of uncertainty about his future, he signed for Ducati's revamped second squad.
In a column for American magazine Cycle World, Spies said the crunch moment with Yamaha came when he was suffering from food poisoning at Mugello.
"Yamaha stayed in Italy and tested the next day, but I didn't ride. I couldn't ride. I couldn't move. I couldn't do anything," Spies wrote.
"A senior Yamaha employee - that's as specific as I'm going to get - said to me 'we've invested a lot of money in you. Don't come to Laguna Seca if you aren't 100 per cent.'
"Then, he added: 'we've lost confidence in you.'
"That was the moment when I decided I wasn't going to ride for Yamaha in 2013. I have a lot of good friends at Yamaha, but when someone talks down to you like that, you lose respect for them."
Spies had a disastrous last season with Yamaha, failing to reach the podium and finishing just 10th in a championship won by team-mate Jorge Lorenzo.
He suffered a run of mechanical failures, admitted he struggled to make his riding style work with the 2012 M1, and then missed the final rounds due to injury.
The 28-year-old said he came close to giving up on MotoGP.
"After my engine blew up while I was running second at Indianapolis, I thought I wanted to leave MotoGP," Spies said.
"I started paying attention to World Superbike. That series was great to me when I was there in 2009, and the previous director, Paolo Ciabatti, is a good friend of mine.
"He would have loved to see me back in SBK. I still might go back there one day."
But despite offers from BMW and Ducati, he ultimately chose to stay in MotoGP, choosing Ducati's Pramac squad ahead of a possibility at Gresini Honda.
"I started thinking a lot and finally concluded that I haven't reached my full capabilities in MotoGP," Spies said.
"What that is, I don't know. I'm not going to say I can win this many races or a championship, but I don't want to walk away and in five years say, 'I could have done this or that.'"
He is optimistic that the Ducati will give him a better chance to show his potential than the Yamaha.
"The Yamaha is a great bike, but it likes a lot of corner speed. If it moves, if the wheels get out of line, it gets upset," Spies said.
"Both the Honda and the Ducati look like they can be ridden a little more 'wild'."
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