Candy is the responsible one out of his pair, and his dog heavily relies on him to survive and simply get around.

In the latter pair, Lennie is like Candy's dog, in the sense that he is very dependent of George, because he is unable to act on his own.

Lennie and Candy's dog are both "non-important" and it is explained that they are easily replaceable.

George and Candy both feel that they have a responsibility to take care of their partner, no matter what is wrong with them, and accept their deaths reluctantly.