The sentence is the site of your enterprise with words, the locale where language either comes to a head or does not. The sentence is a situation of words in the most literal sense: words must be situated in relation to others to produce an enduring effect on a reader.
As you situate the words, you are of course intent on obeying the ordinances of syntax and grammar, unless any willful violation is your purpose—and you are intent as well on achieving in the arrangements of words as much fidelity as is possible to whatever you believe you have wanted to say or describe.
A lot of writers—many of them—unfortunately seem to stop there. They seem content if the resultant sentence is free from obvious faults and is faithful to the lineaments of the thought or feeling or whatnot that was awaiting deathless expression. But some other writers seem to know that it takes more than that for a sentence to cohere and flourish as a work of art.
They seem to know that the words inside the sentence must behave as if they were destined to belong together—as if their separation from each other would deprive the parent story or novel, as well as the readerly world, of something life-bearing and essential. These writers recognize that 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.