Between approximately 2 and 3 years, children begin to understand the logical connection between ideas -- the "why" of things -- which is the reason they start to ask "Why?" about almost everything. It is a big milestone in their development and comprehension of how the world works. However, this stage can also be very confusing and exasperating for parents. It's normal for kids to seem reasonable and wise one minute and act totally irrational the next. This is coupled with the fact that 3-year-olds are still working hard on managing their emotions, which are strong at this stage and can interfere with, and often trump, their ability to act as rational beings.
Other factors will also influence how your child accepts and responds to your logical explanations. Is he tired or hungry? Is he anticipating something? Is he a temperamentally intense, persistent kid? These variables can strain his ability to act (and think) like a "big boy." So when you tell your son he can't have cake for lunch because his body needs healthy foods to grow strong, he may quickly comply. But when you tell him he can't go to the playground before bed, he might completely lose it. You're left confused -- why is one explanation harder to understand than the other? The answer: It's not. That's just how a 3-year-old processes the world.
At this point it is best to explain the rule matter-of-factly and to be consistent in the follow-through. If your son throws a tantrum, validate his unhappiness, anger, or frustration but don't relent as this will just make the tantrum a successful tool for him. It will also confuse him about what the rules really are. When your actions match your words, he will learn the rules much more quickly. Just wait for the deja vu you'll feel in 12 years when you try to explain curfews. Until then, enjoy your passionate 3-year-old and rest assured that understanding logical connections and family rules is a skill he will gradually sharpen over the next few years.