This sonnet is essentially a definition of love. At first, the author classifies love as something that never stops. Love does not change with life's changing circumstances or temptations, it stays the course. Although time will affect the appearance of a lover, that does not change the quality of love. Shakespeare staked his ability to write on the truthfulness of this definition:
If this be error and upon me proved,
I never writ, nor no man ever loved.
When you analyze poetry, you should think about the speaker, audience, purpose, and style of the poem. This means considering literary devices. It seems that the audience is vague, and Shakespeare himself is the speaker. His purpose may be self-exploration and therefore determination of what it means to be in love. As far as devices he used we see personification at work for both Love and Time:
That looks on tempests and is never shaken;
It is the star to every wandering bark,
Whose worth's unknown, although his height be taken.
Love's not Time's fool, though rosy lips and cheeks
Within his bending sickle's compass come:
Here love was given the ability to look, and later to have the appearance of "rosy lips and cheeks". This certainly demonstrates a sign of life which is a further level of personifcation.
In the 3rd and 4th lines, Shakespeare uses word play and repetition and parallel structure with the words:
Which alters when it alteration finds,
Or bends with the remover to remove:
This affects the reader because he/she has to think of a word being used as different parts of speech than it was previously used in the sentence. It feels clever.
Shakespeare is certainly a master when it comes to expression.