It is customary to only use your own toothbrush.
Jerry is late for work and cannot find his toothbrush, but he sees his partner's toothbrush is still in the bathroom.
Jerry wants to brush his teeth.
Jerry goes and buys a new toothbrush and brushes his teeth in the workplace restroom.
Jerry uses his partner's toothbrush to brush his teeth.
Jerry makes it to work on time, and a coworker compliments him on his shiny teeth.
Jerry's partner finds out he used her toothbrush, and she ends the relationship and kicks him out.