Ah, now we get to the "you."
Are we the audience to whom Owen addresses this poem?
We're not quite sure.
Several earlier versions of this poem were explicitly addressed to "Miss Pope," or Jessie Pope, a British propagandist who printed public letters urging men to take up arms in defense of their country's honor.
Owen could be addressing the poem specifically to her.
For the sake of argument, though, let's see what happens if our speaker's "you" is supposed to be us (the readers).
If we accept that we're the people to whom our speaker addresses himself, something interesting happens: we're told that we can't understand what's going on in the poem…even as the speaker tells us what's going on.
In fact, it's like a story that your friend might tell you. They might try to describe something that happened, but then end by saying, "you just had to be there."
These lines actually take it a step further, though: our speaker doesn't even care whether we could actually experience the horrors of battle or not.
He knows that we can't share those experiences with him.
He's just wishing that we could share the dreams of the experiences of battle, but we can't do that.
Such deliberate distancing of the speaker from the "you" of the poem creates a huge gap of isolation in which our speaker dwells.
We just can't understand how horrible his life was…and is.