But it sort of does make sense. You spend your whole life worrying about the unknown. Saving for retirement, dreading what may potentially happen wrt your health. Will I still have a job next year, will I be healthy enough to provide for myself in old age, etc?
In some ways, you're living in a constant state of 'fight or flight', with adrenaline levels at maximum all the time. IOW, you're always stressing about something. When you eventually hear that you're dying, you can basically stop worrying about the future. You only have to think about things from day to day, and hence you can enjoy those days.
I've had that happen with relatives, where in one case they were extremely frugal their whole lives, and never spent money on themselves. When they found out they had a year to live, they went out a spent $20000 on credit cards, and died in debt. But they died (relatively) happy.
And another relative, who was exactly like your anecdote, that became happy once they knew they were dying. It makes sense if you consider that some peoples lives are a constant source of stress, and you can never relax for any reason. In some sense (and this will seem extremely ironic), being told you're going to die is a huge weight off your shoulders. I have to admit that I sometimes feel the same way at only 40 years of age. Perhaps that's not a good sign