To add to the previous post-

Chapter 9 is the pivotal chapter in which Mattie and Ethan make their suicide pact when Mattie told Ethan to "take her down again" (down the slope) so that they could sled down the school hill and hit the Elm tree hard enough to kill them both.

They did this because of the overwhelming fear of losing each other for good, and the prospects of life that were awaiting them- not good at all.

When Ethan agrees to the pact, he asks her to sit behind him. She continuously refuses and questions why he would sit in front. This is where the previous post makes a very good point about the reasons behind Ethan's choice.

However, Ethan's literal answer was: "Because-Because I want to feel you holding me."

From that, we can assume that he wants, in his last breath of life, to feel like a real man. Zeena, his wife, is a emasculating woman: She demoralizes, belittles, weakenes, and discourages Ethan in every way, shape, and form. Mattie is the opposite. She idealizes him.

Therefore, in addition to the previous post, we can also argue that Ethan is experiencing his very last wish: to be a protector to Mattie, to feel like a man in charge, and to do his first and last act of chivalry.

This is actually a great question because there is no textual answer -- you have to consider what he might be trying to accomplish by being at the front of the sled as they try to run down the hill and into the big oak tree in the hopes of killing themselves.  Here are few things to consider:

1. Would Ethan's weight cause greater speed and momentum to create a greater, more deadly, impact with the tree? 

2.  Was it better for him in terms of steering to be at the front of the sled; therefore, he would have better aim for that tree?

3.  Is he really trying to protect Mattie thinking that he would take the brunt of the impact and die, but his body would shield hers and perhaps she would live?

4.  Is he trying to protect her from the gruesome fear of seeing the tree as they crash into it?

5.  Is he trying to make sure he dies and isn't as concerned about whether Mattie does or not?  (a more selfish motive than suggested in #3)

There is no answer to the question.  What is unique about Wharton's novel is that she leaves this a bit open to interpretation -- just as she created the novel with a narrator who doesn't know all the details but directly tells the reader in the chapter before chapter 1 that what follows (chapter 1-9) are his "vision of the story" of Ethan Frome.