Very extreme: extremely happy or extremely unhappy, swinging on a moment's notice from one to another, with nothing much in between. When he was a toddler, DH and I used to call him "bipolar baby," actually
That is DS6, exactly and he's still like that- he's either bouncing off the walls in happiness and the sheer joy of being alive or lying in a heap on the floor screaming with the fiery rage of 1000 suns. And he's loud about everything, he doesn't have a normal tone of voice, everything is done very loudly.
But for the OP, we do use reason with our kids, especially the toddlers. I give short explinations because I truly believe that if kids understand the rules, it makes it easier to follow them. My kids have a "BS" sensor that goes off whenever DH or I make up non-sense rules and they always call us on it. I'm not afraid to use "Because I'm your mother and I say so" (actually it's usually "I've given you my reason, I'm not explaining it again, if you continue to argue xyz will happen"), but only when we're locked in a battle of wills and they're arguing not because they don't understand, but because they don't like the answer.
The boys (DS4 and DS6) seemed to need less reasoning as toddlers, they heard the explination one time, the consequence for not obeying and that was all they needed. DD2 will usually only obey if I explain (again) why she can't do whatever. But, once I tell her she usually simply says "Oh, ok" and then complies.
