Some of the more common reasons insurers decline people for life insurance may include:
1. Age - Over age 80
2. Weight - Obese or Overweight
4. Diabetes - Health problems associated with poor management of your diabetes.
7. Driving Record - 2 or more DUI's in past 5 years.
8. High Risk Occupation or Hobbies.
In addition, there are many more reasons insurers may decline you for life insurance, but that will vary by insurer.
However, some life insurance companies specialize in insuring high risk people for life insurance and will accept diabetics, smokers, and people who are overweight.
And, if you find you have been declined for life insurance you may want to read this article on "Life Insurance Coverage Declined?", which may help you to find life insurance protection.