There are many reasons for heart failure including blockage of heart arteries causing heart attack and weakening of the heart muscle to disease of the heart valves that weakens the heart muscle in addition to various medications and alcohol.

Multiple causes:
Long-standing untreated hypertension (high blood pressure)
Coronary artery disease (Heart Attack) due to blocked arteries.
Infection (Chaga's disease, viral infections)
Excessive Alcohol use
Elicit Drugs
Infiltrative diseases like Sarcoidosis, Amyloidosis
Familial (Rare)

Not just one thing. Multiple causes maybe the cause and his doctor will evaluate the cause.

There are many causes of heart failure, but the most common is coronary heart disease.