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Expressive

There was a reason the Doctor chose to travel with humans, even though there was a myriad of other species in the universe, most of which had life-spans much closer to his own.

He would never admit it, but he had grown fond of the pink-and-yellow creatures over the years.

The main reason he chose them as companions, however, was how absolutely expressive they were. Many of the universe's species hid their emotions and reactions as a defensive mechanism. Humans did no such thing.

With humans, the Doctor got to see their faces light up when he showed them something new, and that joy always made him want to keep showing them more and more.