From writer Gary Lutz:
"...there needs to be an intimacy between the words, a togetherness that has
nothing to do with grammar or syntax but instead has to do with the
very shapes and sounds, the forms and contours, of the gathered words.
This intimacy is what we mean when we say of a piece of writing that it
has a felicity—a fitness, an aptness, a rightness about the phrasing.
The words in the sentence must bear some physical and sonic resemblance
to each other—the way people and their dogs are said to come to resemble
each other, the way children take after their parents, the way pairs
and groups of friends evolve their own manner of dress and gesture and
speech. A pausing, enraptured reader should be able to look deeply into
the sentence and discern among the words all of the traits and
characteristics they share. The impression to be given is that the words
in the sentence have lived with each other for quite some time, decisive time, and have deepened and grown and matured in each other’s company—and that they cannot live without each other."
See full text of his speech to Columbia University, The Sentence is a Lonely Place.