My cofounder quit the day we got our email to interview and wouldn't budge no matter how much I tried to reason, cajole, or beg him to change his mind. At the time I played a weekly poker game hosted by a bunch of YC founders and I still remember walking in with two bottles of champagne in my hand that I bought before I got the breakup text and announcing "guys, I've good news and bad news and reason to drink either way".
I don't blame him at all. We had built up a ton of great chemistry working together for 6+ months but it was on a startup that flamed out with a firesale acquisition and he wanted more stability, especially with his wife grueling away at a residency to pay for their mortgage.
I still think it was a mistake though and not just on his part. He was my Woz and I would've gone to greater lengths to get him to stay if I truly had the Jobs zeal.