"I am happy, that's true," the Brazilian, who lost his Ferrari seat after eight years to Kimi Raikkonen, told the German newspaper Bild.
"I'm glad I went this way. It could have been a stroke of luck. The car feels good," said Massa.
"The fact that Williams changed to the Mercedes engines is the first step to success," he insisted. "I feel that the car is strong and I'm hungry to finally win again."
Massa's other skywards-facing thumb is for the end of the Ferrari era, where despite intense highs and the almost title-winning 2008 season, the 32-year-old has also experienced some deep lows.
Asked by correspondent Nicola Pohl to describe what four years as Fernando Alonso's 'water-carrier' feels like, he answered: "I'll tell you one thing.
"The time as a servant is over! Williams wants me to take a leadership role in the team.
"In a team you sometimes have to do things that you don't understand in the moment. But you're not a racing driver if you cannot win," Massa insisted.