Friendly people tend to lead happier lives because they dwell more on positive experiences, defying the strong negativity bias found in humans, who tend to emphasize negative emotions over positive ones.

Published at the National Institutes of Health, researchers recently asked 200 volunteers to look at a series of negative and positive images for as long as they would like, recording the amount of time each person spent viewing each image. The researchers found that people viewed the negative images longer, but a small set of people preferred the positive ones.

The group who preferred uplifting and pleasant images also scored higher on an agreeability scale, which measured their concern for others, their eagerness to please, and their compliance to other perspectives.

Participants were also asked which kinds of activities they preferred doing: listening to an upbeat happy song or a slow sad one, watching a documentary profiling a famous entertainer or one on government corruption, or a hearing lecture on how to bake a cake versus one on dissecting a body.

"Low-agreeableness participants were equally likely to go for a negative experience as a positive one, whereas the high agreeableness ones showed a strong preference for the positive: anthems, nation's sweethearts and shortbreads."

Sociologists have long documented a negativity bias in humans: we generally feel negative emotions more deeply, are attracted to bad news over good news, and recall negative experiences better than we do positive ones. This negativity bias is thought to be residue from our evolutionary history when quickly evaluating negative emotions, brought on by threat to life and limb, was once absolutely essential to survival. 

As Paul Taylor, Senior Fellow at the Pew Research Center, explains in a fascinating interview, Millennials currently score somewhat lower on the friendliness scale because they tend to be more more distrustful of other people. This may be a result of economic hardship where a more individualist mentality has taken root, or because online social platforms have extended the definition of "friend" without the kind of real-world contact required to obtain real social benefits:

Read more at the British Psychological Society