For what reasons might tooth really hurts after filling?

Not always, but.....: Not all teeth hurt after fillings. In fact, most of the time there is little or no sensitivity afterwards at all. However, think of the process of getting a filling as a minor surgical procedure. The drilling, water, air, etc. Make the nerve of the tooth somewhat temporarily inflamed and this causes some discomfort initially. As the nerve repairs itself, the sensitivity goes away.

Depends on filling: Sometimes white fillings cause sensitivity. If the filling is deep and close to nerve, check with the dentist to take xray and check it. Hope this helps, .