"To be honest, I feel that I needed this change, and maybe it's the same for Ferrari," said Massa, speaking to Portuguese-language publications this week in Sao Paulo.
"When time passes and the situation is the same, you end up losing motivation," he admitted.
"I'm very happy," said the 32-year-old, "especially the way I was received by Williams.
"Since the first day I arrived at the factory, everyone helped me a lot, embracing me 100 per cent and believing in my potential.
"In Williams I am being heard," Massa continued, "but that doesn't mean Ferrari didn't hear me. They listened to me very well.
"Last year I worked on the development of the car, and in the simulator I was practically the only driver, because Alonso wasn't there."
Massa, however, said Ferrari's biggest problem last year was the wind tunnel.
"Many times we saw that it didn't work as it should have," he explained. "At most grands prix we had new parts, but they didn't work on the car.
"We began with a competitive car and ended with an uncompetitive one, which was the opposite of Red Bull."