To facilitate survival [there I go again on this] and continuation of the next generation, all humans are endowed with neural circuit to detect whatever is ‘beautiful’ and whatever is ‘ugly.’
Generally what is systematic, symmetrical, orderly, and other programmed patterns and characteristics are instinctively perceived as ‘beautiful.’
As with all aspects of nature, there are degrees variations, deviations and exceptions.
Thus in some cases due to bad connections in the brain, there is a minority of people who are different from the norm [majority] and perceive the opposite from the majority.
Normally a symmetrical face is deemed to be beautiful and in general this is denote the person is healthy.
Despite the counter views, I still believe the easier processing is ultimately related to health because the genes are processing in a healthy & balanced manner rather than producing say a twisted face.
Sufficiently big [not gigantic] nicely form breasts are beautiful because they are a healthy potential of milk supply for the youngs.
There are many features that are programmed to be perceived by the ‘beauty’ neural circuit as beautiful.
These are the fundamentals and instinctual. This beauty instinct trigger the bonding [primary emotion of love] and attraction/desire towards the beautiful object.
However in many cases, modern man has strayed from the instinctual and developed various tendencies to perceive what is beautiful based on the influence of ‘nurture’ and environmental factors.
Whatever [disorders] is perceived as ugly instinctively will trigger the disgust emotions and avoidance neural circuit set.
Thus to understand what is beautiful [or ugly] we need to first fall back fundamentally on the “survival and procreation on the next generation” instinct and its associated neural circuits in the brain that theoretically exist in all human beings.
As typical with nature we should expect variations, deviations and exceptions from the norm [majority].
Then we do research to understand why there are variations from the norms, it root causes, the neural and other machinery involved and the philosophical and other basis for such variations.