George is loyal to Lennie because he could leave him at any time, but instead he stays with him. He protects him and looks out for him, even though Lennie can be difficult because he thinks and acts like a child.
"We kinda look after each other." He indicated Lennie with his thumb. "He ain't bright. Hell of a good worker, though. Hell of a nice fella, but he ain't bright. I've knew him for a long time." (ch 2)
George protects Lennie from the consequences of his behavior. He tells him what to do, and Lennie usually listens. George is loyal because he could have left Lennie to fend for himself at any time, but he does not.
When George and Lennie are alone, George helps Lennie think of a future by telling him stories of the ranch they will have some day with rabbits. George gives Lennie hope.