I didn't like Arnold
I know I'm going to get a lot of crap for this, but I just didn't like his character. He was just too nice, too, perfect, too good, that it felt lacking and actually got annoying, making him flat out unlikeable.
I remember when I realized this. For some reason, I just didn't like Arnold for the longest time, but couldn't figure out why. Then, a few years later, I was writing a fanficition (not a Hey Arnold one) where I had a few original characters. I had my beta reader go over it, and he pointed out a flaw with my main character, saying that they were too nice that it actually made them unlikeable. I examined this for a while, I realized that they were right.
Later when watching an episode of Hey Arnold (I still liked the show), I was like, OMG, this is why I never liked Arnold. The creators tried to make him as nice and good as possible, that it made him unlikeable (to me), like the writers were trying too hard make him likable that it backfired.
Another thing I really didn't like about Arnold, he was so biased when it came to the stories that go around. If Gerald or his grandfather told a story, even if it contained unbelievable supernatural parts, it was absolutely without a doubt true, no evidence needed. Like the time his grandfather told the story about the ghost in the boarding house looking for his glasses. Absolutely EVERYTHING was proof. There was a creak on the stairs. Yup, that's undeniable proof that the story is true, no questions asked.
Oh, but if someone else told a story with event an hint of supernatural part, there's no possible way it's true. Like the time Sid thought he killed Principal Wartz with the cursed soap voodoo doll, or the time he thought Stinky was a vampire. All the evidence pointed that he was right, but Arnold stubbornly and absolutely refuses to believe it (he was right, but that's not the point). And he's so openly biased and smug about his disbelief, always saying that line, "Well, we could always do so-and-so and find out what REALLY happened." Screw you Arnold, you damn self-righteous hypocrite!