William Wordsworth's Preface to Lyrical Ballads

Line 26: I have said that poetry is the spontaneous overflow of powerful feelings: it takes its origin from emotion recollected in tranquillity: the emotion is contemplated till, by a species of reaction, the tranquillity gradually disappears, and an emotion, kindred to that which was before the subject of contemplation, is gradually produced, and does itself actually exist in the mind. In this mood successful composition generally begins, and in a mood similar to this it is carried on; but the emotion, of whatever kind, and in whatever degree, from various causes, is qualified by various pleasures, so that in describing any passions whatsoever, which are voluntarily described, the mind will, upon the whole, be in a state of enjoyment. If Nature be thus cautious to preserve in a state of enjoyment a being so employed, the Poet ought to profit by the lesson held forth to him, and ought especially to take care, that, whatever passions he communicates to his Reader, those passions, if his Reader’s mind be sound and vigorous, should always be accompanied with an overbalance of pleasure.

Wordsworth explains that every good poem should express some sort of feeling that is conveyed through the words of the poem. Poems are conceived from emotions about a topic or issue in a poet's mind. The reader should get a sense of the emotion being expressed through the mood of the poem. In William Blake's Songs of Innocence and Experience, there are clear moods that each poem takes on. For example:

Laughing Song

When the green woods laugh with the voice of joy,

And the dimpling stream runs laughing by;

When the air does laugh with our merry wit,

And the green hill laughs with the noise of it;

When the meadows laugh with lively green,

And the grasshopper laughs in the merry scene,

When Mary and Susan and Emily

With their sweet round mouths sing ``Ha, Ha, He!''

When the painted birds laugh in the shade,

Where our table with cherries and nuts is spread,

Come live & be merry, and join with me,

To sing the sweet chorus of ``Ha, Ha, He!''

This poem clearly gives off a happy, light and cheerful mood. The poem uses images of laughter and lively scenes to show a upbeat and positive feel to the poem.