2. Oral bacteria
3. Source of substrate (food) that the bacteria turns to acid.
When these three things are available, decay is possible. Every time that you eat or drink, acid is produced for about 60 minutes. That is why it is recommended that you brush after every meal to reduce acid exposure. Failing that, rinsing with water or chewing an sugar
free gum will aid in reducing the risk of decay. Frequent small meals increases the total acid exposure of the teeth and increases the risk of decay. Drinking sodas or coffee with a sweetener through out the day is particularly destructive.
Generally all teeth are similar in their risk of developing decay. Teeth with grooves or fissures tend to accumulate food to a greater degree and thus are the most prone to decay. Teeth with multiple or large fillings are susceptible to recurrent decay due to normal breakdown of the margins of the fillings.
There are some people that have disorders of tooth formation that result in reduced amounts of enamel
or enamel that fractures off the teeth, hence the perception of soft teeth. This is readily recognizable by you dentist and treatment options may be recommended.