In The Fault in Our Stars, Hazel and Augustus said okay a lot:
Hazel Grace Lancaster: Is it really 1 A.M.?
Augustus Waters: Is it? Yeah, yes, it is.
Augustus Waters: [laugh] I should probably go to sleep.
Augustus Waters: [Exhale] Okay.
Hazel Grace Lancaster: Okay.
Augustus Waters: Okay.
Hazel Grace Lancaster: Okay.
Augustus Waters: Perhaps, 'okay' will be our 'always.'
Hazel Grace Lancaster: Okay.
What is the significance of the words "always" and "okay" to the characters and why do they say ”okay will be our always”?
What was the reason behind choosing these words and what was the significance? Why didn't the writer choose another word instead of "okay" and "always"?