I think it's easier to have a set set of rules.
Like on Tuesday DS1 clears up and loads dishwasher.
On Wednesday DS2 does it.
With one of my kids, turned out to have ADHD, they found it very hard to be asked to do something unexpectedly.
They were OK with it when it was their turn, but they had a massive reaction to being asked to do something straightaway. It felt like a sort of threat or an admission of weakness to agree.
Your child isn't a lazy bollocks. Younger kids will just do as they are told.
But for older kids it's like take this example of yourself:
You are in a job, say an office job, and the Boss comes in and asks you to move a load of boxes out of his car for him.
You'd resent that and not want to do it because it's 'not my job' and 'how come I have to do it?'
If your Boss accused you of being mean and selfish and a lazy bad person you'd be upset about that.
Because the Boss got you wrong.
You just didn't see why you got that sudden demand - unexpectedly and out of the blue. That was the issue.
Some people can't cope with unexpected demands.