Pears were a type of fruit.
When the Seventh Doctor turned himself into John Smith, he left instructions for Bernice Summerfield, one of which was to not let him eat pears, as he did not like them and did not want to wake up tasting that. (PROSE: Human Nature)
The Tenth Doctor hated the taste of pears, (PROSE: The Taking of Chelsea 426) at least in ice cream. (AUDIO: One Mile Down) After he turned himself into John Smith in November 1913, he ate a pear as he wandered near his biodata module. (TV: Human Nature) When the Tenth Doctor was restored, he was disgusted by the taste left in his mouth. (PROSE: Twice Upon a Time)
The Twelfth Doctor also warned Clara not to eat pears as they were too "squishy" and always made your chin wet, just before he lost his memories of her. He stated that it was extremely important, and told her to write it down. (TV: Hell Bent) The Twelfth Doctor later gave his future self some advice before regenerating, one of which was "never, ever, eat pears" (TV: Twice Upon a Time) He considered his disdain for pears "his personal taste" and "a quirk born of experience" which he didn't want squandered by any of his future incarnations. (PROSE: Twice Upon a Time)
By the 67th century, Gifftan culture confused fruits such as pears and avocados for a legendary Earth hero called Avocado Pear. Such meant that Yoss Inkl named his son Avocado in honour of the humans Ryan Sinclair and Graham O'Brien who assisted him during his birth. (TV: The Tsuranga Conundrum)
Behind the scenes Edit
In a deleted scene from Human Nature, based on a scene from the original novel, when the Tenth Doctor turned himself into John Smith, he left instructions for Martha Jones, one of which was to not let him eat pears, as he did not like them and did not want to wake up tasting that.
Despite the scene's deletion, this quirk of the Doctor's was carried over into revived series media starting with The Taking of Chelsea 426, and eventually made its way to the TV series in Twice Upon a Time. The novelisation, which, like both versions of Human Nature, was written by Paul Cornell, references the deleted scene.