In his own words
Sonnet 116
Let me not to the marriage of true mindsAdmit impediments. Love is not loveWhich 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 cheeksWithin 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 prov’d, I never writ, nor no man ever lov’d.
The sonnet read at weddings, and stranger than its reputation. It defines love almost entirely by what love is not and never does, then stakes the whole claim on its own correctness.