I'm in the middle of this issue with my ds 7.5-yr-old. Part of it is that he's depressed that there's no one like him around. Part of it is that he's testing boundaries Part of it is that he's quite asynchronous with developments and LOG.
It's definitely related to structure. Some kids need more structure than others or at certain times. It doesn't mean you have to be draconian or have only adult-directed structured activities. It sounds like you're doing much of the same structure that I am - lots of swimming, reading, library time, programs/events at the library, and play dates. That's structure in my book with certain things happening at certain times and days.
I turned to books on defiance (ie. Your Defiant Child) and some structure into the day. This has helped us. I'm not saying it's easy or necessarily needed in your case. But I insist on politeness and respect. When my son recently started get quite angry, frustrated, and having big meltdowns when he couldn't rule the roost or get his way, I locked things up and told him that he had to earn privileges. He's getting the message and connecting his behavior/action and consequences together.
I'm not saying that you've got to be draconian or authoritarian. I'm still struggling with the teeth brushing myself. I even talked about it today with the dentist! It's just that's a non-negotiable rule, though he's free to think how much he dislikes it. My son has to brush his teeth, but he's got control over what toothpaste or what toothbrush he may use. Or he can try to show me how quickly he can brush the teeth. Ditto for putting his shoes on.
