There are many reasons people may be denied for life insurance coverage, including poor driving record, health problems, old age, overweight, smoking, etc.
However, sometimes you may be able to improve your health or change something about your lifestyle (such as quit smoking), which may allow you to qualify for life insurance later on.
In addition, if you are denied life insurance you may want to contact a life insurance broker specializing in finding life insurance for high risk people.
Or, you could try to apply for life insurance with other life insurance carriers.
Another option would be to apply for guaranteed acceptance life insurance coverage which accepts almost everyone depending on your age and state you live in.