In addition to being extremely addictive, nicotine feels good. Nicotine is calming and relaxing while not acting as a sedative. Nicotine also improves memory and may provide some protection against Alzheimer’s disease, Parkinson’s disease, and attention deficit hyperactivity disorder.
Researchers Investigate (Horrors!) Nicotine's Potential Benefits
Of course, the other side effect of smoking - death - cures all known illnesses as well.