Why does Ryuk love apples?

Start with the in story answer, which the manga gives plainly: the shinigami realm is a wasteland, and its apples are withered, rotten things that taste like sand. A human apple, juicy and sweet, is to Ryuk what a drug is to a human: he says as much, shows withdrawal symptoms when deprived, contorting and sulking until fed, and works for apples as wages, searching Light's room for hidden cameras on an apple salary. In a realm with no pleasures, he found the one human food worth haunting a teenager for.

The creator's answer is better, because it is a confession. Tsugumi Ohba has explained that the apples were in the thumbnails from the start for a story reason: Ohba wanted one day to use the dying message that shinigami only eat apples, and therefore needed Ryuk to be holding apples all along. And apples specifically? The red, Ohba said, goes well with Ryuk's black body, and an apple fits his big mouth. When Obata pointed out that apples carry religious and psychological symbolism a reader could read deeply into, Ohba answered that he had not thought about that at all: apples are cool, that is it, with a laugh recorded in the interview.

So the forbidden fruit reading, Eden, temptation, the fall of man, is fan interpretation layered on a color choice. The record gives you a writer planting a prop for a future plot point, an artist noting the symbolism the writer never intended, and a death god whose addiction began as graphic design. It is the most honest answer in the series' whole interview archive, and this page prints it at face value.