Respect is significant in Of Mice and Men because respect largely serves as the foundation for Lennie and George’s relationship. Lennie and George admire and appreciate each other. They try to look after one another and do what’s in their best interest.
Of course, such a statement might seem inaccurate. George is often frustrated with Lennie. He calls him names like "crazy bastard" and speculates how much better his life would be if he didn’t have to care for Lennie. Yet George sticks by Lennie, out of respect for his condition and his Aunt Clara. George tells Lennie, "somebody'd shoot you for a coyote if you was by yourself." The novel's conclusion arguably cements the respect that George has for Lennie. George respects Lennie's personhood and wants him to die humanely.
Lennie, in turn, respects George. He understands the protective role that George plays, which is why he tries not to upset him. Early on, Lennie mentions how nice it would be to have ketchup with the beans. This desire disgruntles George. A little later, Lennie issues a retraction of sorts. Out of respect for George, he tells him he was only kidding around. In fact, he wouldn't eat ketchup if it was right next to him.
Aside from Lennie and George, respect is significant in the character of Curley. One might contend that Curley's lack of respect for himself propels the conflicts between him, his wife, and Lennie. Perhaps if Curley had more respect for himself, he would have treated Lennie and his wife less like objects.