Why does my tooth hurt after filling every single time?

Depends: Some people have nerves in their teeth that are simply more prone to inflammation from any type of procedure. Having a filling done is a traumatic event for the tooth. The deeper the filling, the more likely it is that your tooth nerve will swell and cause pain. Also, if you clentch or grind your teeth, this pain may be present even more.

Post op pain: When a filling is placed, the tooth may develop pulpal hyperemia--more blood flows into the pulp to see what has happened. It is similar to when you stub a toe or finger. It gets engorged with blood and it is sensitive until the swelling goes down. The tooth becomes sensitive to thermal changes and occasional biting pressure. This discomfort should disappear in a few days. Hope this helps.