The actor told the Radio Times that the Time Lord is "attracted" to Jenna-Louise Coleman's "hot chick" companion.
"What's interesting with a new companion is that it changes the way he is and affects his personality," Smith said. "I think, in one way or another, the Doctor is always attracted to his companion and he's certainly taken by this striking young lady."
However, the 30-year-old added that the Doctor will be "quite lonely" and "removed from the universe" when festive special 'The Snowmen' begins.
"The fall of the Ponds had a grave effect on the man," he explained. "I think he's... not really as engaged as he was, at his best with Amy and Rory.
"With my Doctor, Amy and Rory are especially significant... but the show has to propel forward, back into adventure mode."
Photo gallery - Doctor Who filming in London:
Copyright: WENNRichard E Grant (Withnail & I) and Tom Ward (Silent Witness) will also appear in 'The Snowmen', which will air on BBC One at Christmas.
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Watch a trailer for the Doctor Who Christmas special below: