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Moving On
Sarah Jane knew what it was like, traveling with the Doctor. He took you along inside his wonderful ship; made you fall in love with the universe.
He showed you things so fantastic that when he took you back home you could hardly stand to be somewhere so incredibly boring for even the shortest amount of time.
And then, when he leaves you far from home with no way of getting back, you don't know what to do with yourself besides wait for him to return, even though you know deep down that he never will.
There's always so much going on in his head, he forgets things like where he put down a book or when exactly a companion left. Sarah Jane suspected that he realized one day that she was no longer in the TARDIS and assumed that she had left the last time they had stopped in Croydon, instead of him having left her in Aberdeen.
His life was so full of people, always coming and going, it was easy to forget about the humans he took for tours of the universe.