'Why' is actually a question scientists are still asking. There are many different ideas and theories about what causes Alzeimers and other forms of dementias. They do know its due to deterioration or decreased function of parts of the brain due to age or illness. Why this catastrophic damage occures in some people and not others they dont quite know for certain. There is a genetic factor. If members of your family have it you have a better chance, esp if early onset alzheimers (starting in the 40's or 50's) is in you family, of getting it. One theory says its possibly due to depletion of certain chemicals in the brain, esp neurotransmitters.