The Simpsons has gone through a lot of subtle shifts over the years in terms of comedy and substance. While the citizens of Springfield rarely seem to change on a surface level, different directions behind the scenes have morphed plenty of characters in new forms, like the lack of evolution for Apu and the Flanderization of Ralph. That idea extends to some of the central figures in the Fox show.
Homer Simpson has gone through plenty of minor changes over the years. He's transformed from a put-upon everyman into a well-meaning buffoon, and been depicted as everything from a simpleton to a jerk to a hero. But one of his most controversial shifts might have an in-universe explanation, according to a fan theory.
Season 8, Episode 23, "Homer's Enemy" was something of a paradigm shifter for The Simpsons. The episode focused on Frank Grimes, a new employee at the Nuclear Power Plant. A hard-working, no-nonsense employee, he quickly came to hate Homer for his simple but fulfilling life. Despite Homer's efforts to befriend him, Grimes increasingly saw Homer as "everything wrong with America" -- a lazy and selfish jerk who coasted by in life while others were forced into more difficult paths. Grimes' efforts to highlight this discrepancy to the rest of the plant only further endeared Homer to their colleagues, leading Grimes to suffer a mental breakdown that resulted in his death. The end of the episode suggested Homer learned nothing from the experience... but a fan theory hints otherwise.
The theory -- courtesy of the TVTropes WMG page -- posits that Grimes' death might have had a genuine impact on Homer and the way he perceives himself and others. From Homer's perspective, Grimes was an outlier -- someone he couldn't win over with his typical oafish charm and likability, to the point where this development genuinely upset him. His carefree attitude only further enraged Grimes, and all of his attempts to mend that bridge -- either by trying to bring him into his life or gain a professional veneer that Grimes could respect -- were met with anger and ridicule. Homer introduced Grimes to his family, and the man proceeded to belittle him and call him a "fraud" in front of his children, a sentiment the family quietly seemed to agree with. All of that might have convinced Homer that he was a fraud, and if that was the case, he should start acting like it.
In The Simpsons' earliest seasons, Homer was a good-intentioned man. While he made dumb mistakes or acted on selfish impulses, he was always ultimately driven to provide for his family. He sacrificed dream jobs for their financial security and repeatedly endangered himself if it meant protecting them. But after Grimes' dressing down of him, Homer may have quietly decided that he should just start acting like the selfish jerk everyone apparently saw him as. This would give an in-universe reason why the Homer of the next few seasons became crasser, crueler and colder. Nicknamed "Jerkass Homer" by the show's fanbase, this version of the character helped push the show into a different era, playing with a harsher style of comedy.
The Homer who jumped Springfield Gorge to protect Bart in Season 2 and gave up his dream job to provide for his family in Season 6 isn't the same one who framed his wife for drunk driving in Season 15. Yet a version of Homer who'd been broken down emotionally and left more spiteful and selfish might. Grimes was wrong to call Homer a parasite at the time -- but the person Homer became in later seasons fit that description to a tee.
Notably, though, even Jerkass Homer was capable of acts of self-sacrifice, even trading any remnants of his dignity to Mr. Burns in exchange for money for his family. Later seasons would find a happy balance between the more emotional and selfish sides of Homer, suggesting that Homer eventually took Grimes' condemnation to heart and tried to adjust somewhat. It's an interesting theory, one that suggests Homer's transformation into a broader character has a real and poignant explanation.