Why don't Mormons smoke or drink?

The prophet Joseph Smith received a revelation from God known as the Word of Wisdom. It was basically some guidelines for keeping our bodies healthy. What happens when your lifestyle involves smoking and eating junk food all day? You not only put yourself at higher risk for lung cancer, heart disease, diabetes, and other serious medical issues, but you also won't function at your best from day to day. You're more likely to be crabby and ill-disposed to pray or make other good choices. And how about that hangover? I'm pretty sure it's hard to look and feel your best during a hangover.

The Word of Wisdom is just what it implies: a revelation containing some sensible guidelines for our health. 

It teaches that we should exercise, eat healthy foods, and go to bed and wake up early. It also warns us that we shouldn't use or consume harmful substances, specifically coffee, tea, tobacco, and alcohol. (We can still use alcohol and tobacco for cuts and bruises or other skin injuries though.)

God promises us that if we live by the Word of Wisdom, we will find "treasures of knowledge, even hidden treasures" and that we will "run and not be weary, walk and not faint." 

Interpretation issues:

Caffeine

There is nothing about caffeine in the Word of Wisdom. People may consume it if they wish to. Some people have supposed that the reason why coffee and tea are discouraged is because of caffeine, but that's just a matter of opinion. Yes, caffeine can be an addictive substance and therefore probably isn't very good for you. It would probably be wiser not to give yourself the chance to get addicted to it. But it would also be good to never try chocolate for that reason. I guess you just have to choose your battles.

Meat

The Word of Wisdom says to "eat meat sparingly." Most Mormons nowadays eat meat all the time, maybe because the revelation was received at a time when refrigeration was really poor and people got sick all the time from eating meat, but who knows. Some Mormons have taken a vegetarian or vegan interpretation of the revelation. When it comes down to it, the way you interpret and apply it to your life should be between you and the Lord. You should decide for yourself what you think it means and then ask God if your interpretation is correct.