why does ethan tell zeena that he has to deliver the lumber himself, rather than drive her to the flats

Ethan wants to be alone with Mattie.

"Ethan tried to say something befitting the occasion, but there was only one thought in his mind: the fact that, for the first time since Mattie had come to live with them, Zeena was to be away for a night. He wondered if the girl were thinking of it too....

He knew that Zeena must be wondering why he did not offer to drive her to the Flats and let Jotham Powell take the lumber to Starkfield, and at first he could not think of a pretext for not doing so; then he said: "I'd take you over myself, only I've got to collect the cash for the lumber."

Source(s): Ethan Frome