When today's episode aired, I was confused as to why Dragonite was listening to Iris at the very beginning but then became his old self later on. Here's the explanation:
As we all know, Iris's Dragonite loves battling tough opponents such as Mamoswine and Beartic (or Hydreigon), both of which are ice types, a strong opponents for him. Once Iris gave her command, he refused to obey her, establishing the fact that he prefers listening to his own battle instincts rather than those of his trainer.
But, when today's episode aired, Iris gave her command and he listened to her. Why? His opponent was Krokorok, a ground type and a much smaller Pokémon, which led to Dragonite not counting Ash's Krokorok as an exactly strong opponent. He listened to each and every of Iris's commands, both teams exchanged some damage but then it happened. Ash's Krokorok evolved into Krookodile and learned Dragon Claw. The attack struck him hard and was effective.
Dragonite realized how much of a threat Krookodile really was.
It was then when Dragonite started going on rampage, he was becoming his old self. The Pokémon he didn't consider for a tough opponent suddenly got much stronger, evoking Dragonite's battle instincts. He once again stopped listening to Iris, preferring his own method to hers which led to his eventual loss.
What does this mean?
It means that Dragonite is willing to listen to Iris when a weaker opponent is in question, he trusts her in those situations. However, when a worthy and strong opponent is involved, he succumbs to his wild instincts and listens to no one but himself. Naturally, this only became apparent today and the problem still hasn't been solved. That means, in my opinion, that this will lead to some further Dragonite development and hopefully, it will involve Drayden as well.
As this episode has showcased, Iris has a long way to go before she can become a Dragon Master and command over any dragon type with ease. Dragonite has set up an obstacle on her path towards reaching her goal of becoming a Dragon Master so I'm truly looking forward to seeing how Iris will overcome it and grow even more.