I wasn’t trying to justify his attitude, I was saying why it doesn’t bother me so much that we never find a specific reason as to why he’s so standoffish in general. I also think it’s inaccurate to say that he did nothing but make hateful remarks, because as far as other characters go, even those who aren’t respected figures like Cynthia, Paul doesn’t go out of his way to be mean to them. He’s certainly not friendly, but he never insulted Dawn or Barry, and he was capable of holding a civilized conversation with Brock. It’s pretty clear that Ash is the one he had a problem with.
And as far as Ash goes, that did have a reasoning behind it, as was stated in DP186. Those reasons were wrong, Dawn says as much (I’m paraphrasing Hikari’s line from the English subs I have: “Satoshi isn’t Reiji-san. Satoshi is Satoshi”), and Paul acknowledges that she’s right. It was wrong for Paul to antagonize Ash the way he did (and, just to be fair, it would have been nice on some occasions for Ash to have just taken the hint that Paul didn’t like him and quit bothering him), and it’s a part of his character development that he quits doing it and even comes to respect Ash in the end.
But again, I think the show could have given more insight into his dealings with his Pokemon, particularly the way he would release them if he found them disappointing in the beginning. There are things we can infer - I always saw it as Paul not wanting to be like Reggie, who often preached about the same things Ash did (this was said in DP186), and who Paul still hadn't forgiven for his actions after the Brandon battle - but it still would have been nice to see an episode dealing with it.