If you remember Paul form the Diamond and Pearl seasons of the anime, you recall he was a total jerkwad. He treats his Pokemon like slaves and tools instead of friends. He frequently picks on Ash and friends. He borderline disowned his own brother. And he almost drove Maylene to give up training altogether. I've always wondered why he was so mean to everyone he met.
If you want a canon reason as to why, well I can't help you; maybe it was some sort of mentally scarring event in his childhood, maybe he never knew any better growing up, maybe he's that way just because. However, I suspect that the makers of the anime did it to make the audience side with Ash more than the rival character, because why would you support a jerk who abuses his Pokemon? However, this is all just speculation; I doubt we'll ever actually get a real answer.