Shakespeare Sonnet 116

Let me not to the marriage of true minds
Admit impediments. Love is not love
Which alters when it alteration finds,
Or bends with the remover to remove:
O no! it is an ever-fixed mark
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:
Love alters not with his brief hours and weeks,
But bears it out even to the edge of doom.
If this be error and upon me proved,
I never writ, nor no man ever loved.

This sonnet is describing what the true love is. Love is unchangeable because Shakespeare defines that love is a eternal value or truth. When something could be changed or be removed, it will not be fit in the definition of the true love. 

In the second quatrain, true love is like the North Star which can guide the sailors toward proper direction in order to escape from the storm. The height of the star could be known, but the the value of the true love is unmeasurable. The love would not change although human's life is limited. Love is also timeless. It can be tested throughout a long time even toward the end of the world. Love is still love.

The most interesting lines are the last couplet: "If this be error and upon me proved, I never writ, nor no man ever loved." It seems is a paradoxical conceit. It says that if there is no a such thing what the true love is or there is no one has experienced that, he is not going to write it down. However, since the writer has already written the poem down which means what he has said about what the true love is is true.