Smoking is bad for your health, but smoking is also one of the worst offenders when it comes to staining teeth. Tobacco smoke causes brown stains that penetrate the grooves and pits of tooth enamel. Tobacco stains can be hard to remove by brushing alone, and the longer you smoke, the more entrenched the stains become. Smoking also causes bad breath and gingivitis (gum disease), and increases the risk of most types of cancer.
So not just bad for your health, but also for how white & shiny your teeth can be.