Bedtime (again!?)

My children have been going to bed every night of their lives. Haven't yours?

I'll admit that  there are variations in the routine, like whether or not we remember to brush teeth (don't tell the Dentist) sometimes we skip the prayer (don't tell the Bishop). Switch-ups in who gets a snuggle first and which story is read are common.  These days Eden reads a lot of the bedtime stories to the Littles, which I like to call "killing four birds with one stone". 

But more often than not it's a predictable sequence that should go smoothly for these little creatures of habit.

If fact, these children of mine are so familiar with the routine of going to bed that sometimes they play 'bedtime' all day long.  Putting babies to bed, putting each other to bed, singing songs, the whole shebang.  They even build make shift sleeping quarters in various locations, just for the sheer joy of it.

So.  Why is it that bedtime is such a SHOCK when it comes? 

Why do toddlers wail and preschoolers whine?

When the clocks strikes 8pm why does my son lay on the floor and moan and why does my nine-year old suddenly remember the seventeen things that must be completed before school the next day?

Please, please internet.  Please tell me the answers.