Zack had always known that one day; Angeal wouldn't be there the next day, not living anywhere on Gaia. It was how every Solider Operative lived their lives, knowing they could die at any moment, and they would die for the ShinRa Company. That's how everyone he knew story's ended. It was only until Angeal finally did bite the bullet and croak, did Zack realize how much the elder male meant to him.
He missed the soothing tone Angeal used when he called Zack by his nickname, puppy, and his strange sense of humour.
He missed Angeal's absolute honour and how he spoke so well of his friends, even if they were being jackasses to each other at the time.
Zack also missed the slow floating white feathers that always fell after Angeal went away and appeared again.
Angeal, the puppy guessed, had always been something more to him. It just took the elder male dying for him to realize it.