system
Given the following post, respond as if you're commenting on Reddit:
human
[LotR] Why does Denethor hate his son Faramir and love Boromir so much?

Denethor always seems to jump the gun to congratulate Boromir's successes, yet totally ignores anything Faramir achieves and unfairly blames him for outcomes out of his control. Why the vendetta and favouritism?
gpt
There's a few big reasons for Denethor's issues with Faramir:

1. Faramir is close to Gandalf, and Denethor is convinced that Gandalf plans to restore Aragorn/Thorongil (as he was known when he served in Gondor) to his throne, which would essentially usurp Denethor's power.  So Denethor sees this as disloyalty from Faramir.

2. Faramir is more independent and free-thinking than Boromir.  Denethor is convinced that Boromir would have done "what is best for Gondor" and delivered the Ring to him, while Faramir threw away his chance to bring the Ring to his father.  Of course, this points towards Faramir's wisdom more than anything, but to the ruler of a kingdom that's teetering on the brink of destruction, it sure does look like treason.

3.  Denethor just lost his favorite son, the strongest man in Gondor, and his heir.  Now his less-favored son is all that he has left, and he's taking out his grief on Faramir.  And on top of it, Faramir was willing to take on the quest himself, but Boromir went instead, meaning that Denethor *could* have sacrificed Faramir to keep Boromir by his side.  The good person inside of him is likely disgusted by that sentiment, but the grief-stricken madman who's been influenced by Sauron sees only an opportunity that was lost.

4. Denethor's wife Findulias grew ill after Faramir's birth (possibly birth-related complications) and never recovered, dying a few years later.  By all accounts he was completely devoted to his wife and never really recovered from her loss; it's likely that he blames Faramir for killing her.

5. Sauron has been whispering in his ear through the Palantir.  And Sauron ain't known as the deceiver for nothing.  He's great at turning allies against each other and turning small grudges into full-blown hate.  Denethor, mad with grief, was especially susceptible to this manipulation.

6. Faramir is, realistically, a better leader than Boromir, and Denethor knows it and hates it.  His golden son was a born captain and a strong warrior, but he was also headstrong, close-minded, and proud to a fault.  Faramir was just as great a captain of men, but he was also thoughtful where his brother was rash, and he was open to council where Boromir would trust his own instincts.  

So Faramir was at the center of a big maelstrom of issues that his father had, and he was the outlet that his father chose for all of them.  Until, of course, Denethor decided to take them out on himself instead and get a little too fire-happy.