As an audience, we hate the Doctor for not saying it, both here and in Doomsday. But we also love him for it. Because if he says it aloud, he affirms it, he accepts it. And by doing that, he makes it a tangible thing. And he has already lost so much, causing him pain and grief and suffering. So avoiding that four letter word is an attempt to protect himself from the pain of loss. But we know he does this in vain, because feels so much for Rose. And that's what makes this alien a man.