"Professor Elaine Fox, of the University of Essex, said: "People who carried one form of the serotonin transporter gene tended to look on the bright side of life, and selectively avoided negative material." more from The Telegraph.
"Remember the good times
They're smaller in number and easier to recall
Don't spend too much time on the bad times
Their staggering number will be heavy as lead on your mind
Don't waste a moment unhappy
Invaluable moments gone with the leakage of time
As we leave on our own separate journeys
Moving west with the sun to a place buried deep within our minds
And remember the good times..."
The above is from an old Willie Nelson song, a song from back when he was a struggling songwriter. Later the world gained the Willie Nelson band, but for the most part we lost the songwriter. He used to put out a couple of albums a year, with all brand new, wonderfully depressing sad songs. Unfortunately country music wasn't quite ready for Willie and a lot of them have vanished over the years, except in the memory of old farts like me. The above is not on YouTube. I have it on vinyl, but am not interested in mastering the hassle of digitizing and uploading, so here is one from about the same era. Willie before beards and braids - before the years beat the hell out of his voice: