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 wand'ring bark,
Whose worth's unknown, although his height be taken."
(Sonnet 116)
He marveled at the strange blindness by which men, though they are so alert to what changes in themselves, impose on their friends an image chosen for them once and for all. He was being judged by what he had been. Just as dogs don't change character, men are dogs to one another.