why do teeth get really sensitive?

Answer 1

There are several causes for tooth sensitivity. They include:
cavities
bulemia (due to erosion)
acidic food (due to erosion)
chipped tooth
cracked tooth
grinding of teeth
brushing too aggressively (due to abrasion)

You can also experience temporary sensitivity after having some work done such as a filling, crown, etc. It is important to see a dentist since most of these issues require treatment.