Funnily enough, initially I reacted negatively the first time he did it, and years later, I ended up doing the same thing on my Emerald game, rotating off Pokemon that beat the Elite Four so I can train a new set. I can at least say my justification: it gives me more options to use in "important" battles. As for the rest of his traits: stupidity, arrogance, etc., he's not the only shonen protagonist that exemplifies these. It's pretty much par for the genre.So what is it that people hate about him? Is it his team? 'Cause he has a good reason for starting fresh in each region - he likes to challenge himself and build his skills by starting from the ground up every time, and he knows that his other Pokémon will be well cared for at Orchid-hakase's lab.
I kind of wonder why rarely any Ash haters bring up his Marty Stu-ish qualities, because that's my biggest beef with the kid. He ends up being the Chosen one of seemingly every prophecy, he becomes a practical Last Jedi, he died once more than Jesus (not my quote; that's attributed to Blackjack Gabbiani) and this Marty Stu-ness even intruded on May's storyline in the 9th movie, where he had to seal May away in an "Absolutely Safe Capsule" so he could save the day. Then again, a lot of children's TV shows and video games do that kind of thing.