Comments - Hmmm. Unfortunately, since starting this, I've seen a sudden boom in similar-plotted stories. But I've started now, so I might as well get it written and hold onto my denial a little longer.

Summary - Separation is unnatural. A thing that is not whole is broken. 10Doctor/Rose, post-'Doomsday'

Prologue

Separation is unnatural.

To separate a mother from its newborn child is to subject the child to pain, and even death. If you cut skin, it will hurt and it will bleed. People who have had limbs amputated tell of phantom feelings from the missing limb, because the mind cannot accept the sudden loss until experience teaches otherwise.

In nature, there is no such division – only change.

When a child leaves its mother to set out on its' own, their relationship has grown and changed over time to get to that point.

Everything in the universe longs to be whole.

To be broken is wrong, unnatural, against these deep laws. It doesn't matter if something is impossible – everything from life forms to ordinary objects long to be joined. If they are separated through unnatural means, they are broken.

So you see, Rose Tyler has to get back to her universe. Not because she isn't whole without the Doctor (which she isn't). It's not because she needs to hear those three words (which she does). It's not because he needs to hold her in his arms, to give him the courage and spirit to go on (which he does). It's all because of that one grain of glittering light, buried deep within her, that sliver of gold that refuses to let go.

She is the Bad Wolf, and the Bad Wolf must go home.

Separation is unnatural. If you cut off its roots, a plant will not grow. If the universe is not whole, if the Bad Wolf doesn't return?

A thing that is not whole is broken.