If you have pre-existing conditions but are not managing your health as your doctor recommends, then you might have trouble being accepted for life insurance cover. For example, if you have serious liver disease and drink a lot of alcohol, or if you are diabetic and are frequently hospitalised due to failing to manage your diet and treatment, the insurer may decide that you are too high a risk.
The reverse can also be true: if you have experienced a heart attack or stroke in the past, but you can show that you are living healthily to support your health, you could find that you can access reduced insurance premiums.