When NFL players began kneeling during the national anthem, Trump didn't just criticize the players like most conservative commentators; he focused a lot of his rage on the NFL itself, calling it weak and out of control. Which doesn't make a ton of sense ... until you realize that Trump has long held a grudge against the league for refusing to let him buy a team in the 1980s. And when he tried to buy the Bills in 2014, only to get outbid, he reacted the only way he knows how: with shockingly petty tweets about how boring the league was.
And then there's the massive grudge he holds toward his predecessor, Barack Obama. Trump spent quite a bit of Obama's first term cheerleading the birther movement because of, well ... let's say his passion for birth certificate formatting quirks. For some reason, he then attended the 2011 White House Correspondents' Association dinner. There, Obama lit into him. For a solid two and a half minutes, Trump could do nothing more than slowly rock back and forth, tight-lipped, while Obama dished out insult after insult. The guy's probably never had to sit through anything like that before, and the psychic impact it's made on him can't be underestimated. If you're ever in any doubt about the motivations behind Trump's actions as president, know that he'll always do the opposite of whatever Obama would, be that building a health plan, entering the Paris Accords, or reading.