Why does Ethan agree to take Mattie down the hill a second time ?

Ethan agrees because he believes that it will be the last time he feels love and joy.

The spruces swathed them in blackness and silence. They might have been in their coffins underground. He said to himself: “Perhaps it’ll feel like this...” and then again: “After this I sha’n’t feel anything...”

“Get up! Get up!” he urged her; but she kept on repeating: “Why do you want to sit in front?”

“Because I—because I want to feel you holding me,” he stammered, and dragged her to her feet.