The existential attitude is one of involvement in contrast to a merely theoretical or detached attitude. "Existential" in this sense can be defined as participating in a situation, especially a cognitive situation, with the whole of one's existence.
An unlucky rich man is more capable of satisfying his desires and of riding out disaster when it strikes, but a lucky man is better off than him. . . He is the one who deserves to be described as happy. But until he is dead, you had better refrain from calling him happy, and just call him fortunate.