Juve leapfrogged Napoli in fourth place after a 3-2 win over Inter.
"We should have had the same determination and desire to fight on every ball also in the other games of the season," Pirlo told Sky Italia.
"If we had done so, we would not be fighting for a top-four finish at this point. We made too many mistakes in games that seemed easy on paper."
"We've asked ourselves many times what we've lacked this season. We didn't have the same fire burning inside. We had to turn the light on again to fight the title and a top-four finish. The lads have proved they are up to the task in big games," the coach added.
"This group has a lot to give, we have many young players, but playing for Juventus means having more responsibility. It takes time to adapt.
"We shouldn't have dropped so many points, but this is a good team, we have room for improvement, there is a good base to work well."