Smoking causes bad breath, leaves stains on your teeth and tongue, and promotes the build-up of ugly tartar on your teeth. Much of this is due to the fact that substances from tobacco remain in your mouth after you smoke, but some of it may be due to diseases caused by smoking. For example, smoking can cause gum disease, and gum disease can cause bad breath.
Considering smoking's effects on the appearance and smell of a smoker's mouth, it's not surprising that 86% of teenagers say that they would rather date people who don't smoke.