Great Regulars: In the couplet, the speaker implies

that he is so sure of what he has just dramatized about the nature of true love, that if anyone can prove him wrong, then he never wrote and no one ever loved. This assertion places his potential adversary in a very difficult position, for we know he has written, and we also know others have loved.

from Linda Sue Grimes: Suite101.com: Shakespeare Sonnet 116

~~~~~~~~~~~