In Shakespeare's Sonnet 73, the speaker in the poem talks about his fading life. Seasons are often used as symbols for times of life: Spring for youth, summer for life’s prime, autumn for age, and winter for death. Notice the autumnal images with “yellow leaves” and the empty tree boughs where “few” or “none” of these leaves hang. The boughs themselves “shake against the cold,” which signifies the approaching winter.
Other words symbolizing old age, even death, are “twilight,” “sunset fadeth,” and “dark night.” The speaker of the poem shows that he is aged and perhaps even close to death. He addresses a loved one and shows through fire images that his fire will soon go out:
In me thou see’st the glowing of such fire,
That on the ashes of his youth doth lie,
As the death-bed whereon it must expire
Consum’d with that which it was nourish’d by.
The fuel of youth has already burned in the speaker’s fire: youth is now ashes. A fire, nourished by wood, will go out when the wood is burned to ashes--in this sense, the fire itself is consumed in the ashes of what once nourished it.
Finally, the speaker notes that his loved one's love is even stronger because the loved one knows the speaker will be gone soon.