In the beginning of of the book we see the relationship between Candy and his dog unfold.

Candy loves and protects his dog. They are companions, and the dog is integral to Candy in keeping him from loneliness. George and Lennie's relationship is much the same. Lennie is a repsonsibility, but he is also a cherished friend and member of the family. George and Lennie keep each other company.... neither is alone due to the presence of the other.