Over the last three months I organized a lot of overdue home interior fix-ups. Nothing truly major, yet this series of projects has had a surprising mood-lifting effect on me. I used to come home every night to confront "death by a thousand cuts." No, not emotional trauma. I was much more tortured by cracked bathroom tiles, and ice-damaged ceilings, and countless scuffs on our walls. Now I come home to crisp, freshly-maintained rooms (as long as I ignore the day to day mess). I can breathe! But my husband is continually surprised at how openly happy I am about this work. I mention it a lot. Like almost every night. And he must be thinking..."if I only knew the difference a gallon of paint could make."
I suspect that these home improvements stimulate the production of the natural "contentment" chemical substance--oxytocin--for me. There is something about taking good care of my house (and my family by extension) that is mood-enhancing. And I believe that the enormous numbers of women who report the calming, mood-lifting effect of Pinterest to be experiencing some version of the same oxytocin effect.
Why? Take a look at a tiny snippet of my "Gardening" board on Pinterest, below, and an amateur deconstruction of my oxytocin theory, which depends on three key components of oxytocin production: generosity, trust, and optimism:

Generosity: sharing a project doubles its pleasure. In the first two photos I am pinning my spring garden. Coincidentally, I do almost all my gardening in my front yard because I enjoy sharing it. I am away from home most of the week and I like the notion that people are passing by appreciating it in my absence. I feel generous and oddly connected to strangers, which is a documented oxytocin benefit. But now Pinterest enables me to share my garden with, literally, anyone with an internet connection, in a context that makes more sense than, say, Flickr, Facebook, or Instagram. And I get a slice of that same enjoyment in looking at garden pins from other people.
Trust: Pinterest enables people to help each other. In the third pin above, (the purple balls) I am sharing a photo of a garden sculpture I want to make. I am having a little trouble copying what I found in this French garden, so I pinned the photo and asked for suggestions. According to the neuroeconomist and TED talk-giver Paul J. Zak, I just produced an oxytocin moment for the people who follow me on Pinterest. Apparently, one of the central ways to stimulate oxytocin is to give a sign of trust to another person. Zak is publishing a book about it called The Trust Molecule. It's very low-demand, but putting this project up and asking for help enables other people to have that feeling of being trusted. Odds are, I will follow their advice and probably even report back. According to Zak, the mere act of sharing trust is a tiny step to creating a more stable society.
Pinterest is entirely optimistic. Pinboards roughly divide into two categories. One is "here is who I am." (Or at least the parts I am proud of, like my garden.) The second is "here is who I am going to be." This second category is very interesting. It is optimistic and future-facing. Pinning is like losing weight, cooking fabulous food, dressing beautifully, travelling to exotic places, and displaying great taste...with none of the actual effort. This is who I will BE, and no one can argue with that. In my case, I created an "Easter Projects" board full of the charming and obsessive things I would make IF I had time. I am telling the world:
I am really am like THIS person, even though I don't, um, actually have that life right now. But I would if I could, and I will some day.

And oddly, the mere act of pinning these projects, knowing full well I would never make them, gave me half the satisfaction of actually doing them. And as proof (to myself) that these boards can someday be my reality, I used my "Need a new haircut" pinboard to actually figure out a new hairdo. I made this board to show my hairdresser what I wanted, and to solicit input from a couple friends on my finalist choices. So Pinterest can be concretely useful to organizing a project, which contributes to my optimism about the much more unrealistic boards I create. (But for the life of me, I have no idea why 333 people decided to follow my haircut board.)

It's a pretty rare and special thing for a bunch of pixels on a website to provide a consistently pleasant and personal experience. The "oxytocin effect" of Pinterest is not dependent on delivering great new pins every day, or dumb humor, or exceptional stories. I can look at my home page, find nothing of interest, but still wander over to fuss with my own boards and feel...content. I am tending my future. (I have boards for trips I am actually planning, and trips I will never take. Boards for house projects I am doing and houses I will never own. Boards for business ideas, meals, parties, inspirational people, gifts. Basically anything that I can capture online, visually, that I want to remember.) And I probably spend a maximum of ten minutes a day on the site, so it is not as though it takes a lot of work.
Pinterest is the site that gives and does not take. Clearly, being the fastest growing site in history, Pinterest is playing with more than pins and pixels.