A Roulette Wheel and a Piano Playing Dog - S7-E2
Character mistake: Sheldon likes his pretzel and says "Ich bin ein Heidelberger." He makes the same mistake President Kennedy made when he said "Ich bin ein Berliner", which means "I am a jelly donut." The correct German is "Ich bin Berliner" or "Ich bin Heidelberger."
Answer: Some people just have a dislike for someone that can't be explained. Could be as simple as the way he's dressed.