Mr. Shiftlet is a disabled drifter, a wandering tramp with no permanent place to call home. So when he rocks up at Mrs. Crater's farm, he sees a great opportunity to find somewhere to lay his hat. As someone who's disabled, he clearly understands just how difficult it will be for Mrs. Crater to find a suitable husband for her deaf daughter, Lucynell. Shiftlet preys upon Mrs. Crater's difficulties, making himself indispensable. He doesn't really care about Lucynell, or about Mrs. Crater, for that matter; he just looks at them both as meal tickets. Shiftlet's onto a good thing here, and he intends to exploit it for all it's worth. In return for taking Lucynell off her mother's hands, he'll receive a car, some money, and a regular place to stay.
Mrs. Crater, for her part, regards Shiftlet as nothing more than a means to an end. She exploits him by using his skills as a handyman around the farm. Among other things, this allows her to size him up as potential son-in-law material. Looking after a deaf young lady like Lucynell is a bit of a struggle for Mrs. Crater, and so she sees Shiftlet as the answer to all her prayers. Mrs. Crater is not thinking here of what is best for Shiftlet or even for her daughter; she is only thinking of what is in her own best interests.