What Is It About Toddlers And Not Listening?

So the title says it all. Our oldest does not listen to a word we say, or so it seems some days. We can threaten taking away his most precious toys or send him to the time-out corner but it doesn't seem to make a difference. Some days (okay lets be honest, all days) I am frustrated beyond belief. Our conversation goes something like this... "(Bulldozer), can you please pick up that toy you threw? We do not throw toys." Then he makes the face that lets me know that he is not listening to a word I say! Blank stare and unmoving facial expression. "(Bulldozer), get off of your brother! We do not sit on him." I then become on the receiving end of some really evil looks, including the sullen stare that says, "I don't have to listen to you." Every once in awhile he listens to my instructions or commands without argument. At those times I wonder what I've done right at that moment and try to replicate it so I can achieve success next time. It never works... I know that he isn't the first toddler to disobey his parents but really, is it this hard for everyone to get their little one to listen?