I learned a very important lesson about jealousy driving back from J. C. Penny’s last night.

David and Katherine had just picked out 15 new pairs of underwear (I’m on day three of potty training Katherine, if you hadn’t already picked up on the tension in my recent posts), and a pair of summer flip-flops.

“Katherine,” David said to his younger sister in the same tone of voice as when he asked me why good people get shot by soldiers, “now that you have flip-flops, other people will get mad at you…because they don’t have flip-flops.”

David warned Katherine of the burdens that those with flip-flops must endure, specific dangers that she may encounter now that she had joined the elite flip-flop clan.

These beach shoes could destroy friendships in a single foot movement.

“If Mimi comes up to you and takes your flip-flops, then she is no longer your friend,” David explained.

“Yeah,” Katherine replied, “’cause she is just mad that she doesn’t have flip-flops.”

“And people who have different kinds of flip-flops might get mad, too,” David said. “Zane is wearing the kind that Nana gave me last year. But these (looking down at his toes) are much cooler. So he might try to steal my flip-flops. But then he is no longer my friend.”

This flip-flop conversation lasted fifteen minutes, covering every possible angle of envy and jealousy.

I couldn’t help but contemplate my brain and Mrs. Smiley.

You see, Mrs. Smiley–a fellow preschool mom with two smiley kids–is never without a wide grin (with the cutest little dimples). I see her at the gym, pumping iron and smiling (yes, that’s possible); holding her kids at fundraisers and smiling (as she bids on the ridiculously priced “abstract” painting that her Monet children apparently worked on); tossing boxes of Kashi cereal into her grocery cart–where her two smiley children sit placidly (telling her “broccoli, Mom, more broccoli!”) — with a bloody smile on her face.

Along with the smile, of course, are the positive sentiments. In the same vein as those uttered by the guy in front of me at the coffee shop today.

“Another fantastic day to be alive!” he said to the barista, who, I hope, poured him decaf.

“How are you, Frank?” she asked him. (Why would she do that after his introduction?)

“Happy as a coconut!” he responded. (I had no idea coconuts were so emotional.)

Like Frank, nothing negative ever escapes the lips of Mrs. Smiley.

This bothers me.

Because I’m jealous.

She’s got cool flip-flops and I don’t.

My theory is that her prefrontal cortex–the brain area just behind the forehead, which is home to the executive branch of emotions–looks a little different and runs a touch more smoothly than mine. The research of Richard Davidson, Ph.D., professor of psychology and psychiatry at the University of Wisconsin, show that many depressed people appear to have a power failure in the left part of the prefrontal cortex (shown in PET scans and electrical studies of brain response).

I know from positive-psychology studies (Martin Seligman, author of “Authentic Happiness” and others), that the brain is largely plastic, and that we can make ourselves happier with some effort (gratitude, helping others, yada yada yada). However, we were born at a certain happiness level (genetic programming), and that’s our starting point.

It’s equivalent to weight loss and metabolism.

Exercise and diet certainly contribute a great deal to determining a person’s BMI (body mass index) and shape, in general. But everyone arrives on this earth with a different set of genes that substantially influences her physique.

There are those who are very blessed (I’m jealous of them as well) with breakneck metabolisms, who stuff themselves with chocolate Krispy Kreme doughnuts, bags of Doritos, and Burrito Supremes from Taco Bell–whose only work out is maneuvering the remote control of their TV–and have nice figures. And there are also the unfortunate lot who were born with extra wide hips, flabby legs, and full chins. They eat salads for lunch and dinner (no dressing) and run 30 miles a week, and they still wear plus sizes.

As far as my weight, I’m somewhere in the middle. I try to eat healthy, and work out a ton. I go on occasional binges, and then have to diet. I certainly have to work at my figure, but not nearly as much as those friends of mine whose parents are overweight and unfortunately passed on the wide hips and flabby legs.

But when it comes to my happiness level, I feel like the obese chick eating nothing but raw vegetables (no dressing) and working with a physical trainer for three hours every afternoon, but still shopping at plus-only boutiques. If I didn’t have to think before I opened my mouth every time, almost everything I said would be cynical and sarcastic. Thanks to Seligman and the happy doctors, I now have an edit feature inside the noggin, where each thought is sent to get fitted for a positive cap.

But it’s very hard work, training every single thought. I get tired of this boot camp inside the left side of my prefrontal cortex. Sometimes I just want to coast–to wear a smile without forcing it. Like Mrs. Smiley seems to.

My exhaustion at times turns to jealousy. And I’m tempted to steal Mrs. Smiley’s flip-flops.

But then again, all this cognitive weight-lifting inside my head is good for something. It is largely responsible for my wit, and allows me to see the comedy in practically any situation–even a conversation about flip-flops. The amount of energy I expend to revise each of my thoughts translates into a sense of humor that I can flex in the face of such disgustingly cute grins as Mrs. Smiley and her smiley kids.

Because, as Francoise Sagan (was she without flip-flops herself?) said, “To jealousy, nothing is more frightful than laughter.”