This is a (free access, download 9-page pdf) paper by RIM Dunbar of primatologist fame. Along with Robert Sapolsky I've learned more about human nervous systems from him than I ever knew from studying PT.
Studying PT is being trained in how to use a toy pail and shovel. A lot of effort goes into explaining what shovels do and what pails are for, and how to scoop and dump, but almost no emphasis is placed on the vision of the huge expanse of beach one has been set loose to tackle, or what kind of castle to build, or the nature of sand itself, or how long one can expect to have to dig, or how easy it is for the sand castles to be washed back out to sea.
Once out on the beach, one generally heads over to where other castles are already under construction, and gets a job helping. Sometimes PTs head off to a secluded part of the beach and figure out how to make their own castle. Some PTs go into the business of making and selling ever more attractive pails and shovels, pointing out the inadequacies of the ones that exist. Many never ever get that building castles is futile altogether.
Long ago I threw away the pail and shovel, walked down the beach to the water, went into it, and figured out how to swim. I took all the time needed to learn properties of water, learn to float, swim, with and without goggles, learn to avoid sharks and other predators, and how to stay warm even as water sucks at my core temperature. It's been a very engaging life; even if it doesn't look like it has amounted to much from the outside, on the inside I'm very contented and feel fulfilled, all that. I love being a human primate social groomer, swimming around in other peoples' nervous systems, most of the time, most days.
Dunbar points out that (abstract) "Grooming is a widespread activity throughout the animal kingdom, but in primates (including humans) social grooming, or allo-grooming (the grooming of others), plays a particularly important role in social bonding which, in turn, has a major impact on an individual’s lifetime reproductive fitness. New evidence from comparative brain analyses suggests that primates have social relationships of a qualitatively different kind to those found in other animal species, and I suggest that, in primates, social grooming has acquired a new function of supporting these. I review the evidence for a neuropeptide basis for social bonding, and draw attention to the fact that the neuroendrocrine pathways involved are quite unresolved. Despite recent claims for the central importance of oxytocin, there is equally good, but invariably ignored, evidence for a role for endorphins. I suggest that these two neuropeptide families may play different roles in the processes of social bonding in primates and non-primates, and that more experimental work will be needed to tease them apart."