Happy is far, and far stereotypes more, pays less attention to detail, cares less, and feels higher status:
Very high levels of positive feelings predict risk-taking behaviors, excess alcohol and drug consumption, binge eating, and may lead us to neglect threats. … Those who early in their lives reported the highest life satisfaction (for example, judging it at 5 on a 5-point scale) years later reported lower income than those who felt slightly less merry when young. What’s more, they dropped out of school earlier. … A group of American college freshmen who in 1976 claimed to be very cheerful. Surveyed again when they were in their late 30s, they earned, on average, almost $3,500 a year less than their slightly less cheerful peers. …
When we are sad, we think in a more systematic manner. Sad people are attentive to details and externally oriented, while happy people tend to make snap judgments that may reflect racial or sex stereotyping. … Those in a happy mood were more likely to find a fellow student named “Juan Garcia” guilty of beating up a roommate than one identified as “John Garner.” The control group was pretty much equally divided between “Juan” and “John.” … Some of the students received a picture of a middle-aged, bearded man; others of a young woman in a T-shirt. Even though the essays were identical, those students who had been induced to feel happy judged the man’s work more competent than the woman’s. Their non-induced colleagues declared both essays to be of equal quality. …
Cheerful people are easier to deceive, couldn’t detect lies as easily as those in negative moods and couldn’t tell a thief from an honest person. … Feeling good makes people more selfish (if asked to divide raffle tickets between themselves and others, they’ll keep more in their pockets than sad people) and worse at defending their opinions (they produce weaker, less detailed arguments). (more)
There is probably (as is so often the case) that you have an inverted U with happiness / longevity / success in life.
At the depression end you do not get anything done, suffer physically and may even commit suicide. At the eternal bliss end you do not do anything either (and thus are likely not to be much of a success / live a healthy life) so slight unhappiness that gives you drive forward is probably not such an absurd optimum state?
Fair enough. Except that, if there's a positive selection effect, you'll end up with an entire population of bad decision-makers.
Great Filter? :)