The reason Sawyer cares so much for Locke and trusts him that he will be back no matter how long it takes is because he and Locke have a special connection - Locke's dad was Sawyer's enemy and when Locke told Sawyer that he wanted his father dead as well he finally was able to fulfill his promise that ha made to this "Sawyer" con-man since he witnessed his parent's death as a child.