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-fixèd 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 is imperfect for many, and varies according to people’s different natures. Some can see inner beauty in a plain person. However, love remains an ‘ever-fixed mark’, that is, it is consistent. This could be likened to the Pole star that guided sailors at the time. 
 There is a break, or  caesura , either at the end of the previous line or after the  exclamatory  ‘O no!’, depending on how one interprets this.  The reader has to pause at this point and think of the meaning; a little dramatic climax before the main dramatic climax later. The line is short and abrupt, to emphasise its dramatic importance. The next three lines are smoother and refer to the comment about the ‘ever-fixed mark’. 
 Shakespeare, having described flawed love, states an emphatic negative, before proceeding to describe the ‘tempests’ that can buffet and damage love, asserting that true love can resist these.
Sonnet 116