Larry David, creator of the hit sitcoms Seinfeld and Curb Your Enthusiasm, hails from Brooklyn but has lived most of his life in Los Angeles. On a rare stay in Manhattan to film episodes for Curb – in which he plays himself – David went to see a ball game at Yankee Stadium.

During a lull in the game, cameras sent his image up to gigantic Jumbotron screens. The entire stadium of fans stood to cheer him.

But as David was leaving later that night, in the parking lot someone leaned out of a passing car and yelled "Larry, you suck!"

On the way home, Larry David obsessed about that one encounter: "Who's that guy? What was that? Who would do that? Why would you say something like that?"

It was as though those fifty thousand adoring fans didn't exist – there was just that one guy.

Negativity focuses us on a narrow range – what's upsetting us. A rule of thumb in cognitive therapy holds that focusing on the negatives in experience offers a recipe for depression. Cogitive therapy treatments might well encourage someone like Larry David to bring to mind his good feelings when the crowd went crazy for him, and hold his focus there.

Positive emotions widen our span of attention; we're free to take it all in. Indeed, in the grip of positivity, our perceptions shift. As psychologist Barbara Fredrickson, who studies positive feelings and their effects, puts it, when we're feeling good our awareness expands from our usual self-centered focus on "me" to a more inclusive and warm focus on "we."

Focusing on the negatives or positives offers us a bit of leverage in determining how our brain operates. When we're in an upbeat, energized mood, Richard Davidson has found, our brain's left prefrontal area lights up. The left area also harbors circuitry that reminds us how great we'll feel when we finally reach some long-sought goal – the circuitry that helps keep a graduate student slogging away at a daunting dissertation.

At the neural level, positivity reflects how long we can sustain this outlook. One technical measure, for instance, assesses how long people hold a smile after seeing someone help a person in distress or after watching an exuberant toddler prancing about.

This sunny outlook shows up in attitudes: for example, that moving to a new city or meeting new people is an adventure opening up exciting possibilities – wonderful places to discover, new friends – rather than a scary step. When life brings a surprising positive moment, such as warm conversation, the pleasant mood lasts and lasts.

As you might expect, people who experience life in this light focus on the silver lining, not just clouds. The opposite, cynicism, breeds pessimism: not just a focus on the cloud, but the conviction that there are even darker ones lurking behind. It all depends on where you focus: the one mean fan, or the fifty thousand cheering ones.