Rasmussen's Law of Parenting
Over my past few years of parenthood, as I have observed my emotions, I have noticed a strong correlation between where we currently are in the spectrum between "Idyllic Family Photo Album Moment" and "Screaming Frazzled Crazy Parent Birth Control Commercial". It's all a matter of how much of a hurry I'm in and what all else needs to get done, e.g. cooking, cleaning, errands. The idea coalesced into a single sentence a year or so ago, which I think I may have tweeted, and, as I go about my daily life of raising my kids, it comes to mind. Every. Single. Day. 
Lazy Sunday mornings reading children's books and long Saturday afternoons at the playground are on the left; mornings getting ready for school are somewhere in the middle; and packing for a vacation and preparing the house for a dinner party are on the right.
Every parent's curve will be slightly different, depending on how authoritarian or high strung they are, but I'm pretty sure the slope will be negative for everyone.
The concept is both descriptive and prescriptive; it explains both why you are unhappy and how to be happier. When you feel like you're one spilt juice away from offspring defenestration, it allows you to take a step back and reevaluate your unrealistic ambition to simultaneously draft that corporate memo and live in a house without legos and chocolate all over the floor.
Don't get me wrong, many times you just don't have a choice of where you are on the x-axis. For instance, your toddler isn't going to understand why Mommy has to work two jobs to pay the rent. And sometimes you just have to let your child cry for your attention while you cook her dinner or take a shower or clean up her brother's vomit. It can be very easy, however, to get too ambitious and either not seek out enough childcare help or have overinflated standards of household cleanliness.
What I love most about it is that it removes blame from the child, which is a philosophy that can be very easy to forget when you're rushing the family out the door and your son keeps taking his pants off. Personally I find it very useful to be reminded, on a daily basis, that all the stress in a parent-child relationship comes from the parent.
Parents, what do you think?
I only put my name on this sentence because it seems like the kind of statement that should exist on the Internet in "[somebody]'s Law" webcomic form. Feel free to share or modify it. 