Why does Shiftlet abandon Lucynell?

It is a grim reality that Shiftlet does not love Lucynell. To Shiftlet, Lucynell is just a means to an end. Accordingly, Shiftlet abandons Lucynell because his main goal of obtaining an automobile has been accomplished.
The text tells us that Shiftlet uses flattery to trick Lucynell's mother into giving him what he wants. Shiftlet also presents himself as a man who has fought bravely for his country and who still cherishes traditional ideals. To further endear himself to Lucynell's mother, he sets about making repairs to the property; he fixes a fence, the roof, and the steps to the house. 
Impressed by the prospect of having such a grand son-in-law, the unwitting older woman consents to Shiftlet marrying her deaf-and-dumb daughter. She even gives Shiftlet money for a weekend trip and pays for paint for the automobile. Unfortunately, Shiftlet later abandons Lucynell at a diner, telling the lone worker behind the counter that he is on his way to Tuscaloosa. Now that he has the automobile, Shiftlet has no further use for Lucynell: this is why he leaves her behind.