Because the human mouth has about the same bacteria count as the human anus.
Fun fact….
“ Toilet seats have less germs than mouths! It is estimated that toilet seats have 3,200 bacteria per square inch, where as saliva has an estimated 100 million microbes of bacteria per mililitre with anywhere between 400 and 600 different species. Fact 5: Your mouth encounters more germs than the rectal area!”
Imagine what the biter’s girlfriend/whateverfriend is thinking. No more kisses.