My son is 8. We had very few structured rules until he was 3. Up to that point he was very flexible and prosocial and tended to do what we were doing or something happily compatible. At 3 he began making unreasonable demands, like a totally different meal that would take substantial preparation when we were already eating. Although there were, and still are, SOME times when we could negotiate briefly and come to a mutually acceptable solution, more often he would drag out the negotiation trying to get even more than he'd originally asked. Sometimes his negotiating behavior becomes so obnoxious (claiming he should get some reward because a parent supposedly mistreated him in some unrelated way, and/or loud or nasty tone) that the discussion gets derailed into trying to get him to behave acceptably. I get annoyed by this, and my partner gets really really upset--we feel disrespected and feel he's being ungrateful for what he already has--so we've prevented some of it by setting rules that are not negotiable.
With a different kid, this wouldn't be necessary. The one we've got functions better with rules once he gets used to each one, IFF we stick to it like glue. If he can catch us making an exception, he'll suspect it's not really a rule and challenge it multiple times a day. These aren't polite, reasonable challenges, more like following us around the house nagging shrilly, interrupting every time we respond, and making various threats about what he'll do if we don't let him have his way. Furthermore, when he gets really worked up, he'll actually freak out at getting what he wanted, saying it isn't good enough or we don't love him or something--which tells me that he does not truly want control of this decision but needs us to exert authority.
The first rules we made were a few months after he first started resisting going to sleep at night. It wasn't important for him to go to bed at a certain TIME so much as to have a bedtime that didn't take three hours of focused parental attention, involve kicking me in the face, and/or result in anyone screaming at anyone. Finally we had a meeting where we wrote up an agreement about what Mama will do, Mama will not do, Daddy will do, Daddy will not do, Kid will do, and Kid will not do. We posted this in his room at his height (although he couldn't read) and for several months each of us noted when one of the others was not following one of these rules. I wish we hadn't had to do that, but we did, and it helped a lot.