"Reverie" means a state of being lost in thought or daydreaming, which aligns well with the metaphor of life being fleeting, like a morning.
"Actuality" refers to reality, which doesn't fit the metaphorical sense of the sentence.
"Proximity" means nearness in space or time, which doesn't fit the context.
"Truth" refers to fact or reality, but does not convey the fleeting or ephemeral quality suggested by "morning."
Thus, "Reverie" is the best fit.