Secondary(reparative) Dentin (revised May 12, 2005)
Generally, the answer is irritation.
It's sort of like callous formation on skin.
If the irritation is serious enough inflammation occurs.
If the case of teeth secondary dentin is laid down by odontoblasts in the layer of the pulp immediately adjacent to the dentin lining the canal or pulp chamber.
Sources of that irritation are usually heat, cold, occlusal pressures and the aging process.
One academic endodontist of my acquaintance once told me that if people lived long enough all of their teeth would become pulpless and most would require endo somewhere along the way.
An endondontic lecturer once made the offhand remark that endodontic anatomy hasn't changed much in the last 10,000 years and probably won't change much in the next 10,000.
WRONG!
It has changed dramatically in the last 30 years alone.
The reason was not DNA but GNP.
Our democratic capitalistic society has extended the life span of people by controlling disease and aging causes secondary dentin to form.
As we keep teeth longer, canals get smaller and harder to treat.
As more restorative work is done, canals get irritated and get smaller.
As periodontal disease is treated, more root surface is exposed and when that type of irritation occurs it is further toward the apex and instead of having to break through the dentin occluding the orifice we have to dig through sometimes 10mm of totally occluded canal to find a patent canal.
Furthermore, people are better dentally educated and want to save teeth and with more insurance and disposable income, they are capable of paying for it.
Endodontics is NOT going to get easier in the future, it is going to get harder.
Techniques and instrumentation of the past are not longer able to serve us.
We have to meet change with change.