Having a "I've connected the dots" moment on why Zeus is so affectionate with Apollo, and I don't know why I'd never considered it before that Apollo's youth could be a reason.
Iamblichus, The Life Of Pythagoras (trans. Guthrie)
So there was this sentiment that young boys are the dearest to the gods, and the gods are especially attentive towards their requests. And since Apollo is a perpetual young boy himself, with Zeus being the one to grant him this gift in one version, it makes sense that Zeus has granted so many of Apollo's wishes. He spared Prometheus and Periphas upon Apollo's request even though he was personally offended by them. He agreed to suppress Gaia and Athena upon Apollo's request when it came to the oracular business. He even revived Asclepius to console Apollo despite Apollo's frankly disrespectful reaction to Asclepius' death.
Apollo might be fearsome, powerful, and even reckless at times but when he kneels down in supplication, all Zeus sees is his little boy that he loves so much, and he can't help but give in to his requests (well, most of his requests at least). And as far as the text is concerned, and if I'm interpreting this correcltly, Apollo's youth is interestingly tied to his helpful nature because besides using his own powers to help the humans, Apollo can also help them by supplicating to Zeus on their behalf, as Zeus is more likely to grant a request when it's coming from him.
