You can see yourself in brown eyes. The old saying about gazing into the abyss, except that the void staring back is so much deeper than you could imagine, a burnt, polished copper flecked in gold. Your reflection, glimmering in the light, the edges of a smile crinkled on one side and the beginning of tears on the other.
For that is the gift. Eyes of green or blue may be windows into the soul, stained glass through which the self may filter; brown is the soul itself, raw and real and human. Brown is the universe, the vast gulfs of space that let such brilliant lights shine, the solution to Olbers' paradox that lets us look up at the stars and wonder.
Brown eyes are human, and it is what makes them beautiful.