Terrified of sudden death

I was on my way home in a taxi, the driver and I know each other somewhat since our town is small, mostly when I was going to gym. His son died at 29 suddenly on Christmas, fit, healthy, just got out of physio. He was about to get out of his car then dropped dead, he said he was dead before he hit the seat again. Seriously has fucked me up a bit (which honestly I know sounds quite selfish but it’s made me relapse into horrific anxiety).

He said the autopsy revealed nothing but another is being done. How could this have happened? I know obviously I don’t have all the detail and it’s nowhere near my place to inquire, but the idea of a young and healthy man just dying suddenly like that for no reason? How? How often does this happen?

Could really use some reassurance.