Form Constants

These basic visual forms are composed of 'Shape Constants' and can cause human FAPs: Facial Awareness Patterns, i.e., visual attraction patterns on the face that draw the attention of bystanders. Shape Constants are visual forms that our nervous system can easily perceive through our eyes and retain in our brain's memory. As a result, we notice such entoptic forms better. Unconsciously, we find an interplay of lines attractive because these forms emphasize each other's prominence.

If you were to compare our perceptual ability to a recording from a digital video camera, you may have seen the following: Low-resolution video images look 'blocky'. This blocky structure of the image is an intrinsic property of the digital camera's hardware. Similarly, our nervous system has intrinsic properties that ensure we can perceive certain images better than others.
The configuration of our neuronal circuits has genetically evolved in such a way that it can 'capture and retain' certain simple figurations due to the way our neural circuits are built (intrinsic pattern matching). These features in a female face 'catch the eye' of men who are attracted to these women with these kind of natural beauty.

However, many women nowadays accentuate these optical features using mascara, eyeliner, lipstick, blush, etc., to attract men's attention.
This artificial, external beauty treatment has an unfortunate side effect on facial evolution: it is not genetically reproduced! So, mascara masks the real (underdeveloped) FAPs that are genetically propagated to their daughters. So, wearing facial make-up will slow down the development of natural beauty among women of future generations.

Because women with clear / striking human entoptic features are noticed at an early stage of courtship, the chance that a man will remain attached to her may increase.

He becomes 'hypnotized' by the beauty of her face. He will likely want to marry her and have children with her. And if she retains that visual beauty, he will stay with her longer to help raise their children.
With beautiful women, the chance thereby increases that these human entoptic forms will be repeated in their biological daughters. Beautiful women can have beautiful daughters. And with a beautiful married couple (husband and wife), the chance increases even more that their children might become even more beautiful.
In this way, the neural circuits regarding human entoptic forms will gradually evolve (via genetic reproduction) and improve their complexity over multiple generations. (This evolutionary phenomenon is called: a positive intrinsic sexual selection mechanism.)
Of course, beauty is in 'the eye of the believer', so beauty is a very subjective trait; however:

I dare to assert that most people share a common basic appreciation for certain features of beauty. This can be seen especially in films starring precisely those actors and actresses who are perceived by most viewers (statistically speaking) as beautiful. This fascination for beauty causes viewers to keep watching the film. Consequently, such viewers unconsciously promote a film featuring beautiful people.

Metaphysics of Beauty

So, beauty of the face is a subjective appreciation by the observing person.
But this person may choose the reproductive partner, to have children with, based on his/her internal appreciation of external facial beauty.
This way, both the 'subjective experience' as well as the 'facial shapes that seem beautiful' (FAPs), propagate through time, from generation to next generation, during evolution of humanity.
Effectively, 'physical beauty' as well as 'beauty-appreciation' become intrinsic properties of the human race.
The aesthetic experience of facial beauty internalizes into the genetic blueprint (DNA) of mankind.