Low and rustic life was generally chosen, because in that condition, the essential passions of the heart find a better soil in which they can attain their maturity, are less under restraint, and speak a plainer and more emphatic language; because in that condition of life our elementary feelings co-exist in a state of greater simplicity, and, consequently, may be more accurately contemplated, and more forcibly communicated; because the manners of rural life germinate from those elementary feelings; and, from the necessary character of rural occupations, are more easily comprehended, and are more durable; and lastly, because in that condition the passions of men are incorporated with the beautiful and permanent forms of nature. 

For all good poetry is the spontaneous overflow of powerful feelings: but though this be true, Poems to which any value can be attached, were never produced  on any variety of subjects but by a man, who being possessed of more than usual organic sensibility, had also thought long and deeply. For our continued influxes of  feeling are modified and directed by our thoughts, which are indeed the representatives of all our past feelings; and, as by contemplating the relation of these general  representatives to each other we discover what is really important to men, so, by the repetition and continuance of this act, our feelings will be connected with  important subjects, till at length, if we be originally possessed of much sensibility, such habits of mind will be produced, that, by obeying blindly and mechanically the  impulses of those habits, we shall describe objects, and utter sentiments, of such a nature and in such connection with each other, that the understanding of the being  to whom we address ourselves, if he be in a healthful state of association, must necessarily be in some degree enlightened, and his affections ameliorated. 

*M.H. Abrams's definition of DECORUM

Decorum, as a term in literary criticism, designates the view that there
should be propriety, or fitness, in the way that a literary genre, its subject matter,
its characters and actions, and the style of its narration and dialogue are
matched to one another. The doctrine had its roots in classical theory, especially
in the versified essay Art of Poetry by the Roman Horace in the first century
B.C. It achieved an elaborate form in the criticism and composition of
literature in the Renaissance and the Neoclassic age, when (as John Milton put
it in his essay Of Education, 1644) decorum became "the grand masterpiece to
observe." In the most rigid application of this standard, literary forms, characters,
and style were ordered in hierarchies, or "levels," from high through
middle to low, and all these elements had to be matched to one another. Thus
comedy must not be mixed with tragedy, and the highest and most serious
genres (epic and tragedy) must represent characters of the highest social
classes (kings and nobility) acting in a way appropriate to their status and
speaking in the high style. A number of critics in this period, however, especially
in England, maintained the theory of decorum only in limited ways.
Thomas Rymer (1641-1713) was an English proponent, and Samuel Johnson
(1709-84) was a notable opponent of the strict form of literary decorum.
See neoclassic and romantic, poetic diction, and style, and refer to Vernon
Hall, Renaissance Literary Criticism: A Study of Its Social Content (1945). Erich
Auerbach's Mimesis (1953) describes the sustained conflict in postclassical Europe
between the reigning doctrines of literary decorum and the example of
the Bible, in which the highest matters, including the sublime tragedy of the
life and passion of Christ, are intermingled with base characters and humble
narrative detail and are treated with what seemed to a classical taste a blatant
indecorum of style. For Wordsworth's deliberate inversion of traditional decorum
at the beginning of the nineteenth century, by investing the common,
the lowly, and the trivial with the highest dignity and sublimity, see M. H.
Abrams, Natural Supernaturalism (1971), pp. 390-408.