I think what characterises all the D's are their ability to max out on their dreams. They pursue their dreams with such a passion it's almost like they're "drunk" of their dreams and passions. That's one unifying quality I see among all the D's which is why they're not afraid. They have a strong inner conviction and fortitude that makes them pursue life to its fullest, living life with no regrets. They're all funloving happy people but they're also a little crazy, you know. It's the people we get drawn to.
The reason why the D's die they smile when they die it's almost to show that they have conquered death by conquering life itself. This is why they smile when they die. They have no regrets.