Disclaimer: I do not own Doctor Who.
He never could understand why humans did it. It left a bitter taste in your mouth and it didn't really smell nice and it was, in fact, slowly killing you. But smoking was supposed to calm you and relieve your worries for a short period of time, or at least that was what people said, and exactly what he needed right now.
When he had entered the TARDIS after Wilf had promised to look at the stars every night for him, the Doctor could feel it, creeping up like poison in his body, the gnawing feeling of loneliness, the realization of all that had happened in the past couple of hours. He had stayed in the TARDIS for a bit, just staring at the console, motionless, until he couldn't take it anymore, he had to let it out, wanted to scream and run, tired of pretending, tired of being 'fine'. It was as though he was suffocating.
That's when the Doctor thought of it, the thing said to be an escape. He went a month back in time, on an eventless night in Cardiff and stepped out into the cool air. The Time Lord sluggishly walked to a nearby ATM and pointed the sonic screwdriver at it, memories of Donna coming flooding back. He really didn't like stealing but perhaps Earth could spare a couple of fivers for him.
The Doctor bought the first pack and lighter he saw in the little shop and went outside. The streets were empty which was ideal. He tore the packaging, pulled out a cigarette and lit it, then took a long drag and slowly exhaled. He wanted to get lost in the cloud of smoke around him, to forget.
So he wandered around the darkest Cardiff streets until the sky became the colour of the tip of his last burning out cigarette.
A/N: Smoking is awful, guys, don't do it.