From fairest creatures we desire increase,
That thereby beauty's rose might never die,
But as the riper should by time decease,
His tender heir might bear his memory:
But thou contracted to thine own bright eyes,
Feed'st thy light's flame with self-substantial fuel,
Making a famine where abundance lies,
Thy self thy foe, to thy sweet self too cruel:
Thou that art now the world's fresh ornament,
And only herald to the gaudy spring,
Within thine own bud buriest thy content,
And, tender churl, mak'st waste in niggarding:
Pity the world, or else this glutton be,
To eat the world's due, by the grave and thee.
This can be re-phrased as “Take pity on the world or else be regarded as a selfish glutton. By the laws of nature you must have a child so all your loveliness does not get taken at the grave.” 
 Note the change in metre at the beginning of line thirteen, with “Pity the world”, the emphasis being on the first rather than the second syllable of “pity”.  This changes the pace and alerts the reader to the conclusion. 
 The conclusion resolves the poem in the final  couplet , with a rather gloomy statement that changes the tone from what has gone before.  The reader is left wondering what the young man’s reply will be.
Sonnet 1