I just had my new guy that I really pushed to hire quit after only 3 days. The hiring process was rigorous and he seemed like a great fit! How do you prevent new hires from quitting during onboarding?

I just had my new guy that I really pushed to hire quit after only 3 days. The hiring process was rigorous and he seemed like a great fit! How do you prevent new hires from quitting during onboarding?

He interviewed well, including an interview with our CEO, and had all the right credentials. Our onboarding process is pretty average. He said when he left that he didn't think our company is right for him. Anyone have experience with this??