Could you explain why does it hurt when you get fillings in your tooth?

Flexure ; Acid: Drilling irritates a tooth, so you may have discomfort afterwards. If you have white fillings, acid was used before placing the filling; if this acid lingers, then you will have discomfort. Also, white fillings change shape slightly when they solidify, so they can pull and flex the tooth, causing discomfort.

Tooth Nerve: The nerve inside the tooth becomes irritated when it is stimulated by the restorative procedure. That's why a local anesthetic is often used.

Somatic pain: A painful stimulus can be mechanical, thermal or chemical and must be of sufficient intensity to induce somatic pain during dental treatment. The first step when getting a filling is to make sure that you are pain free throughout the procedure. Dentists usually do this by using a local anesthetic. No worry.