Cuz they start stinking after a while, even tho the fix isn't that difficult: little access door at the bottom opens up, pull out the filter, which is probably mega-clogged, and clean it.. when you pull the filer, which unscrews on mine, be prepared for a lot of water to gush out...
Friendship is like peeing your pants.. Everybody can see it, but only you can feel the warmth..