What theme is introduced by Ethan's pondering his ancestor's gravestone?

The cemetery has always mocked his desire for freedom: it is full of Fromes who never escaped from Starkfield, and Ethan used to feel that the graves promised him the same fate. That night,as he walks with Mattie, he feels content with the idea that he and Mattie might stay at the farm together forever, but his fantasies are untenable.