There are a number of images of true love's stability in the sonnets. Some of them deny its stability; some celebrate it.
The two most famous are probably found in sonnet 18 and sonnet 116.
Sonnet 18 starts "Shall I compare thee to a summer's day?
Thou art more lovely and more temperate"
and ends with "
So long as men can breathe, or eyes can see,
So long lives this, and this gives life to thee. "
These claim the beloved's beauty does not change like the seasons, and that the poet's love will outlast time and vision.
In sonnet 116, Shakespeare says of love "It is the star to every wandering bark," saying that love is so steady you can steer a boat by it.