Day 1: Birth
When Natsu Dragneel was born, his brother was fascinated. In fact, he'd already been for about seven months. Following their mother around just to feel Natsu kick, fashioning little toys out of scraps of cloth and enchanting them as best he could.
But the second he saw the little boy's pink hair- which, as soon as Natsu could talk, Zeref knew, he'd tease his younger brother about endlessly- and arms punching at the air, he knew that no matter what he did, he would always love the boy.
Even though Zeref might not look like him, or act like him, and, as the years went by, would prove to not be as brave or reckless, he was always his little brother's protector. Held him back from fires which he insisted he ought to run into to prove his worth, stopped him from climbing the trees whose branches were just a hair too thin to hold him. He was always holding him back, but every time the pink-haired boy grinned up at his big brother and told him he loved him.
And then, all of a sudden, Natsu was gone. Not left, like Zeref did when he went to the magic academy. Gone, like someone had snuffed out a candle that burned bright with sakura flame because its light hurt the eyes. He'd shone too bright, and he was gone. But his brother had sworn to protect him, so Anhkseram be damned he would.
He was cursed. Not just by the fact that everyone he loved died. No, that was just an afterthought, now. Really, truly, honestly cursed because he'd loved his brother too much. Natsu had shown him just how much he could really love life, and now the only hope he had left to him was gone like so much cloud across the skies. He'd never be able to bring the little brother he loved so much back to life, because if he did then all it would do was force Zeref to watch the only thing left he cared about die.
Natsu stirred, yawned in a way that showed off his none-too-human fangs, and finally (finally!) breathed, living after years trapped in lacrima. Zeref had been fascinated by him for years, and the second he was reborn, the older boy knew that his little brother was the most perfect thing he'd ever see.