Rockets, Communists, and the Dewey Decimal System - S1-E2
Other mistake: As Young Sheldon is a prequel series to Big Bang Theory, this episode is at odds with episode 2-13 of Big Bang Theory, "The Friendship Algorithm," in which Sheldon is unfamiliar with books like "How to Make Friends and Influence People," which he reads in this episode of Young Sheldon, produced nine years later but taking place twenty years earlier.
Babies, Lies and a Resplendent Cannoli - S5-E18
Other mistake: Georgie tells Mandy he'll soon be 18, in March. That same day, Sheldon says Isaac Asimov died - which happened on April 6, 1992.
Ants on a Log and a Cheating Winker - S7-E4
Other mistake: Sheldon (in the voice-over) says "it really brought what's his name and I closer together." The grammatically correct expression is "brought what's his name and ME closer together", and a stickler like Sheldon would not make that mistake.
Answer: Some people just have a dislike for someone that can't be explained. Could be as simple as the way he's dressed.