I bought a house 5 years ago. About once a year I have to take my bathroom sink drain apart and clean it. My wife's is fine.
At first I thought it might be from cleaning my electric razor into the sink, so I stopped; but it hasn't helped.
All that goes down it is toothpaste and fluoride rinse, and it goes down my wife's also.
Its like a combination of rotted vegetation and cheese.
Hey, I warned you that it was disgusting.