What are some reasons why life expectancy is so short?

The primary negative factors on life expectancy in developed countries are, in order of most negative influence to least:

1. Transportation-related fatalities. I.e. car, bus, train, and airplane deaths and serious injuries.
2. Poor diet - diets which significantly increase risk factors for cardiopulmonary diseases, and also increase the risk of Diabetes.
3. Air pollution - particularly from coal-burning and wood-burning
4. Domestic (i.e. inside the country, not inside the home) violence; i.e. homicide in all forms. This risk correlates strongly with prevalence of guns in a society.
5. Spotty use of available medical care, particularly preventative medicine.
6. Genetic factors in a population - the particular genetic makeup of a population determines which diseases it is most predisposed to.

In developing countries, the negative factors on life expectancy are:

1. No access to clean drinking water
2. Poor sanitation and personal hygiene
3. Low levels of vaccination
4. No access to common "basic" medical care
5. Air, water, and environmental pollution
6. Fatal accidents (as a result of no access to trauma care)
7. Warfare

In all cases, it appears that human genetics enforce a maximum lifespan of 120 years, and that under optimal conditions, the majority of humans will die around 100.