The Doctor got bored without Rose, and rather lonely, so he decided to get himself, not only a new companion, Donna, but also a little one of his own.

One nursery and artificial womb later, a baby, you. The next day, a toddler, the day after, a kid, the age you'll be staying until your next regeneration. You are a miniature version of the Doctor, after all. His pride and joy.

Today, you're in bed, fast asleep, now a child, the Doctor quietly entering to wake you up. He perches on the edge of your bed, brushing a gentle hand over your hair, speaking softly "Wakey wakey, rise and shine."