My dentist always told me I have good teeth and now my teeth are falling. How can this happen?

You should know that your teeth may be succeptible to periodontal disease and not succeptible to tooth decay  (caries). So probably when  your dentist told you that you have good teeth, ment that your dentition is not succeptible to caries. In other words  it is possible that people with periodontitis do not have problems with tooth decay and vice versa.