Feeling Rebellious

Why is it that when someone says you can't do something, that little switch goes off inside and says "I want to do that."?  I know I'm not the only one with this interesting switch.  In fact, I witness it in my children all of the time.  If I say "no" to going to the park, they become fixated on going to the park.  A "no" to having ice cream, prompts a meltdown.  More interestingly, though, is when it's something they don't normally desire to do.  For example, I was sick a few days ago and my 4 year old asked me to work on school with him.  I said, "not right now."  This caused him to bawl because he really wanted to do school.  This also prompted my 2 yr old to bring his school book to me and ask to "please do school mommy?"
In any case, the switch is there and has once again been flipped.  I've been feeling rather lazy the past 2 weeks, only working out a handful of times.  This meant nothing, however, until my OB told me to put my feet up and rest - no working out.  Now I can't seem to get the gym from my mind.
Even though I know it is best for baby and I right now if I rest, and I am resting... there is still that little part of me that wants desperately to go for a walk or better yet, hit the elliptical.  Will that switch stay flipped when the baby is born and I have the all clear to work out again?  I hope so, though I doubt it.  There is something extra special about doing something you are not supposed to do.  Come to think of it, maybe I should go tell me kids they are not allowed to clean their rooms.  I wonder how well that would work... :)