Why does my tooth hurt

Sensitivity to cold (especially air), more so at the gum line.

Likely cause:
Receded gum & exposed tooth root. This is often the result of incorrect tooth brushing technique with a too hard bristled toothbrush.

Why does it hurt?
The tooth root surface is normally sensitive to air, touch, cold drinks, etc. Fine nerve endings in the dentine (tooth root material) only send a message of pain to the brain if they are stimulated. It is not fair but that's how it is!

What can I do?
If the area is decay free (they usually are) - desensitising toothpastes often help. If they do not help or decay is present, small restorations (fillings) with a tooth coloured adhesive filling material (e.g. glass ionomer cement) often solves the problem.

Sensitivity to cold and possibly sweet - more so between teeth.

Likely cause:
Early - intermediate tooth decay (Dental Caries)

Why does it hurt?
The decay (caused by bacterial action) exposes areas of dentine to the oral environment. Like the exposed tooth root discussed above, anything that stimulates the fine nerve fibres in the dentine (cold liquids, sweet solutions or foods) will send a pain message to your brain.

What can I do?
Call your dentist. Treatment usually involves removal of the infected tooth tissue and placement of a restorative filling material (a "Filling").

Sensitivity to cold and hot items.

Likely cause:
More advanced tooth decay. Possibly associated with a cracked tooth.

Why does it hurt?
The Pulp (what you might call the Nerve of the tooth) is inflamed because of the deep dental caries (decay process). The inflamed pulp sends pain messages to your brain when stimulated. Treatment to remove the decayed tissue is required.

What can I do?
Call your dentist. A large or deep restoration (filling) is usually required. Because of the possibility of damage to the tooth pulp, this tooth will need to be reviewed. The pulp may not recover. If so, Root Canal Treatment will need to be considered.

Sensitivity to cold, hot and pressure on tooth.

Likely cause (Option 1):
A cracked tooth.

Why does it hurt?
As above - The pulp is inflamed because of the crack. Biting on the tooth either allows the crack to open or pushes the tooth roots into inflamed areas around the root tips. The pain tends to linger on after your stop the stimulus.

What can I do?
Call your dentist. You will need to discuss treatment options for this tooth (or teeth) that almost certainly will involve root canal treatment or tooth removal.

Likely cause (Option 2):
A non-vital (or dead, infected) pulp, commonly called an "abscess".

Why does it hurt?
When you bite down, you push the tooth roots into inflamed, often infected bone tissue around the root tips. This problem often hurts the most over-night, waking you up with severe pain.

What can I do?
Call your dentist. The only treatment options are root canal treatment, or the removal of the tooth.

Sore, bleeding gums.

Likely cause:
Gum disease.

Why does it hurt?
The inflamed, infected gums are letting you know they are not in a healthy condition.

What can I do?
Call your dentist. A thorough cleaning of all your teeth in probably overdue.

Sore, bleeding gums, wobbly teeth.

Likely cause:
Advanced gum disease.

Why does it hurt?
As above, the inflamed, infected gums and bone supporting your teeth are letting you know they are not in a healthy condition.

What can I do?
Call your dentist. Gum disease leads to tooth loss.