It was actually a great narrative twist. XY Ash was bordering on stu throughout the entire season. His team, his battles, his donut steel frog...He had it all...What could possibly go wrong? Well. That's just it. Ash was doing so well that he forgot how to lose.
In previous seasons he always had someone around to keep his ego in check. Misty and Brock were his coaches early on. Gary and Paul pushed him as rivals in their respective seasons. Even Iris to a lesser extent bullies Ash and reminds him that he's "just a kid" But XY? Oh, XY...He's surrounded by groupies in this show. So when he finally loses a couple of rounds with donut steel, he throws a tantrum and shuts himself off. Battle Bond failed because he was becoming impatient and obsessive. He even yelled at his waifu and told her to go away because "You don't know how I feel! Waaaah!". Like a baby. Ash was losing sight of himself. This is why he needed to lose again, and hold his head up high...
