Letters involving Zack and Aerith play a prominent role in the developing back-story between the two character. In one instance, you can view the letter Zack sends to his parents in the international version of Final Fantasy VII. Upon clicking the object, you read that Zack apologizes for leaving town so abruptly in order to become a SOLDIER. In the post scriptum ("P.S.") section at the bottom of the letter, he writes "I got a girlfriend" (Japanese:ガールフレンドができました。) . Though realizing his dream of becoming a SOLDIER is important, Zack suggests that the addition of Aerith in his life is something also worth mentioning.
The letter found at Gongaga in home of Zack's parents (Final Fantasy VII International)
Towards the end of Crisis Core, Zack discovers that Aerith had written him 89 letters over the course of four years while he was imprisoned by Hojo. Although she wasn't aware of his capture, Aerith still had faith in her love for Zack; even if he didn't respond, she still continued to write in order to preserve her feelings for him. Upon discovering the letter carried by Angeal's clone, Zack realizes how much time has passed, and read that Aerith would not continue to write anymore letters. Consequently, he cries out in agony before quietly saying "Aerith... wait for me." As his last mission, he decides to set out for Midgar in order to see her and fulfill her small wish, but is shot down just beyond the city's outskirts.
As shown in the letter sent to his parents where he talks mostly about becoming a SOLDEIR and only squeezing in a sentence to mention Aerith, Zack loses her through the very same problem. He places more emphasis on the career aspect of his life, placing his job before her; it is almost obvious that Zack deems his job more important over Aerith, even if he doesn't mean to. Because of this, Aerith's only wish was to spend more time with him, which would have been enough for her. In the end, Zack finally rearranges his priorities correctly by deciding to protect Cloud from the very job he had pride in being part of, and more importantly, to go see Aerith in Midgar. However, it was too late; although he fulfilled his former duty, he would not survive to finish the last one.
In a sense, there is an underlying tragic message in that destiny decided the two would not be together; their relationship was dragged down by sad coincidences and twists of fate. On a less monumental scale, no matter how outstanding people Aerith and Zack may be, or what kind of extraordinary situations they are placed in, they still had a typical, human relationship; it just happened to be under very special circumstances. The irony in this is not that it was destined to fail by some freak chance; there were also precise hints in the game telling us that it was coming undone all along. But this is what makes their relationship natural because in real life people who fall in love don't always get their fairytale-ending. Relationships, just like people, aren't perfect and can fall apart from flaws and misunderstandings. Though their relationship may have ended with a letter, we are still privileged to watch them together from the moment their love blossomed, and all the way through.