Their hypothesis is that as social animals, we are wired to respond neurologically to status Higher/lower status individuals experience dramatically different levels of certain neurotransmitters, which is something that they cite animal studies to support. Experimentally manipulating the social environment e.g. by placing a monkey in cage where it was surrounded by either smaller or larger peers caused observable changes in brain chemistry as the monkey moved from alpha male status to subordinate and back again.

There's a good argument for an evolutionary role here, as you can see that being on the bottom of a social hierarchy may be dangerous, so those who experienced it as stressful and were motivated to escape it somehow may have had higher chances of survival.

Being trapped in such a position appears to cause a deeply stressful state of mind, where people are literally unable to function normally. This doesn't happen when people are more equal as the perceived status differences are small, so the effect is lessened.