Here you'll find a short speculation without anything baking it but would be not to far fetched ( I think/hope )
So ten minutes ago I was watching the marineford arc, and was at Ace deaths. I don't know why but when I saw his last smile, I just thought about the 942 chapter with it's revelation about the smile.
And here what I end up thinking : The D have been curse or bless or both
At their death they end up in a fruit, the devil of the fruit. Their happiness being the only thing that stays.
But how, why, and by who?
My guess would be by using Uranus, the Roman god of life. In my head if it wants to be powerful, it must control life then. And with this supposition it would make sense that it can keep a part of someone from dying. Furthermore in the myth his d*ck was thrown to the see increasing the link towards the DF and the curse of the see.
Why and who ?
It's either someone against the AK or someone from the AK.
Someone against the AK:
IMU or one of his/her ancestors
In order to curse the D from having a peaceful after life or he/she knew about the DF results, but needing strong soul/will/emotions, he/she cursed the strongest warriors of her time the D.
Someone from the AK (joy boy ?):
They were the strongest at the time perhaps they cursed themselves to grow stronger eating the DF telling themselves we got only one life who cares,... But don't think so seeing how no real strong D (Roger,Garp) has a fruit how garp reacted when he remembers about Luffy's fruit. The same goes for Roger teammate, and shanks and buggy perception of the fruit (like disgust of it) ah and the Admiral getting easily the best one.
But still perhaps joy boy if the fruit is an after effect and what intended was to have their will to stay in the world. So at death he put it in a fruit to stop it from disappearing of the world (seeing their imminent defeat)
But it could also be a side effect from Imu's curse, the karma kicking his as*
Oopssy it was longer than intended. Well, hope you've enjoyed