Toulon president Mourad Boudjellal conceded that Castres "were hungrier" than the European champions after Saturday's shock result in the Top 14 Final.
Just two weeks after claiming their first silverware in over two decades, Toulon saw the tables turned on them by the rank outsiders in a 19-14 defeat at the Stade de France.
Despite the loss, Boudjellal insisted it was a season to savour.
"If you'd have told me at the beginning of the season we'd finish as European champions and reach the Top 14 final, I wouldn't have believed it," he said.
"We're disappointed because we weren't far away (from winning both) but I don't have any particular reason to get angry, not like last year (when Toulon lost the final to Toulouse).
"They won it within the rules. When I saw Castres arriving for the warm-up I saw they were hungrier than us. We had something to eat not so long ago, we weren't quite as hungry.
"Maybe it's because we're European champions that we lost. We were lacking some gas, freshness. We made mistakes in our fluidity."
Toulon's wait for a fourth French title will now run into a 22nd year but former England fly-half Jonny Wilkinson says they will learn their lessons and come back stronger next season.
"We have to continue with the same spirit next season and learn from these lessons," he told France 2 television.
"We didn't play well enough to win, we made some mistakes and we lacked tempo.
"They scored during their good moments and deserve to win. We'll work hard and do everything to win next season."
What had been a turgid slugfest in the first half was brought into life on the stroke of half-time as Rory Kockott flummoxed Toulon's defence with a dummy pass before scything over under the posts.
With a seven-point lead at half-time, Castres were in the ascendancy and although two Wilkinson penalties briefly brought Toulon to within a point, Castres' management of the second half was clearly superior.
A pair of drop goals from fly-half and captain Remi Tales helped put them out of sight before Delon Armitage's consolation try after the final hooter had sounded.
But according to English lock Nick Kennedy, it was the Kockott try that really hurt.
"It's always bad to concede just before you go in, it does change your mentality a bit because you feel you're chasing the game whereas if we'd just closed it off after that scrum, we go in with our heads up," said the former London Irish second row.
"The timing of that was bad for us and extremely good for them."
For Kennedy, the game boiled down to small details, where Castres were better at turning their pressure into points.
"It was a very close game, they took their chances well. Those two drop goals midway through the second half were timed very well and executed extremely well.
"They're a good side, a very good team, they were more fresh than us in the ruck. In the first half their lineout was very good, their scrum decent and we couldn't get a lot going in attack. From one to 15 we all made mistakes and didn't really get our game going.
"You have to give them credit for their defence, we tried to vary it but they were very good in the ruck and if we didn't get there in numbers quickly, they'd turn us over. It's something they must have done a lot of work on and they're very good at it."