The double world champion, whose new deal was announced last week, has been instrumental to the British-based team's march up the grid after Canadian businessman Lawrence Stroll took control in 2018.
"When you see how (Alonso) is working, when he is there, how he is interacting, you see that there is only 100%. When he says there is no life around that, it is true," Krack said.
"Considering that, I had some concerns that he was going to say: 'You know? I want to do something else with my life,' and I was very happy to see that he loves Formula 1 more than private life at this stage."
Alonso moved to Aston Martin for the 2023 season from Alpine and secured six podium finishes for his new team.
Targeting at least one more world championship before retiring, Alonso's name had been in the mix for a seat at Mercedes for 2025, following seven-time world champion Lewis Hamilton's departure to Ferrari, or champions Red Bull.
Krack attributed Alonso's decision to stay with Aston Martin to his belief in the project, with the team having an exclusive Honda engine supply deal from 2026.
"I think what we have here is (Alonso's) desire and absolute passion and even rage to win," Krack said. "Which many, many drivers at this stage of their career do not have."
Alonso's decision to stay with Aston Martin narrows the options available to others looking for a drive for 2025, with Stroll's son Lance the Spaniard's current teammate.
"Aston Martin is Lance's home, we know that, and the whole project has always been around him," Krack said with regard to the Canadian driver's future.
"We look for continuity. I have always said that is very important."
Alonso praised his teammate for sometimes being "more sensitive to things... which is very important for the team" in terms of car setup in comments at a press conference ahead of Sunday's Chinese Grand Prix.
"I think the analysis that Lance can reach and can feed back to the team is crucial to us and to improve the car," he added.
