I was discussing the plausibility of Luna and Noctis being so into each other with someone a while back when her being the Oracle and 'destined' to aid Noctis as king was brought up and hammered home the idea that their split was so devastating and lingered because of the destiny shared between them and the 'normal' relationship forged in the middle, along with the brutal way they left each other's company. They are two people forced to carry the same global burden by fate as opposed to volition -- so when they met for the first time and ended up enjoying the other's company not as an oracle and king, but as friends, it left a clearly strong human impression. They wanted to see each other because they liked each other, even if the stars had deemed it so, anyway. They came from the same place, and were going to the same one, too -- but when it came down to it, they were also two instant close friends aware of their status as royalty who were simply interrupted by political pursuit. This is why their fondness for each other persisted throughout the 12 years and transcended the simple implications of magic-dog note transmission acquaintanceship by actually being the attraction of 2 magnetic forces obstructed by the walls of war, both prophetically and naturally meant for each other, but denied such by circumstance. This complicated things and muddled the line between friendship, loyalty, and romantic love -- they didn't ever get the opportunity to really draw it, but because they were so important to one another as prescribed by fate, they couldn't ever leave the other's mind.
In other words, their relationship seemed abnormally defined by their predetermined destiny, the circumstances that split them, a lack of closure, and liking one another as ordinary people, so wasn't your usual pen-pal armlock.
The problem is the game doesn't translate this well at all. It doesn't show you any of those extra layers through consistent in-game conventions, just hints at 'em. The flashbacks come suddenly and pass by quickly without selling us on how meaningful they are, and when Luna ends up dying, what could've been an interesting subversion of our expectations (being the 'girl you want to reach but can't') ends up being just another fridge moment which Noctis gets sad about sometimes.
But I do think their relationship could've been a very interesting one if it was just told with more clarity. No it's not your usual love-story, and that's what was so exciting about it for me. And then ch. 9 ended and I was like, 'that was it?' xP