Finding the perfect word for a line is like picking the perfect blossom for a flower arrangement. The right flower depends on a variety of factors: amongst others, its significance, shape, size, colour, texture, perfume and impact; and so does the right word.
One must consider its definition (literal or figurative), connotations, evocativeness, rhythm, sound (for example, its alliterative power) and composition (spelling or letters)—in fine, its ability to complement, enhance and complete the poetic bouquet so as to eloquently embody and convey the theme.
What is more, in a traditional poem, this is true for every word—in every line. Traditional verse is a delicate composition of the written and spoken word—not an amorphous outpouring of thought or emotion; it requires every ounce of ingenuity from the poet to bring into being, even in its simplest incarnations.