Historically, some have stereotyped the British as being reserved and having bad teeth. To the extent there is some truth here, could one have caused the other?
Maybe a person with bad teeth is prone to not smiling and speaking loudly. He might come across as reserved.
Alternatively, maybe a person who is reserved has less incentive to care for his teeth. Potential mates may not care as much, if the teeth won't be often seen.
What may have happened is that these two traits survive better as a pair than each in isolation. A symbiotic relationship, like, um, a lichen.