My ds is 6 and generally pretty brilliant - well behaved at school, makes friends easily, etc. He has a pita younger sibling who pushes his buttons but it takes a lot to make him fight back. He also likes to to play by himself for long periods of time - lovely imaginative games with characters etc using playmobil people or lego but this doesn't;t worry me as he likes playing with his siblings or friends too. But when he fixates on something it is like his brain gets stuck - usually this is something in a shop but could just be wanting another biscuit or to go to the park. He will talk about it, convinced himself it is owed him, start whining and just not snap out of it however many times he is told no or has the reasons why not explained to him. It is literally like his brain gets stuck. It can sometimes take a good half an hour or more to move him on from it. Is this just normal or something developmentally we need to look into it. I have two other kids and they have never done this - I mean they want things of course, but they don't get absolutely stuck on it.