1. The complexity of the extraction procedure which can lead to more trauma in the surrounding area and hence more resulting pain and extended healing time.
2. Food build-up in the socket
3. A remnant of a root tip or loose bone chip (something that naturally can occur in all cases of extractions) may be working its way up and needs to be removed or,
4. The occurence of a dry socket - a condition which needs to be treated immediately but not life threatening.
In order to rule out which of these exist, though, I would suggest you return to the doctor that removed your tooth so that he/she can properly evaluate . They will do this by taking an X-ray of the area and directly looking into the extraction site and the surrounding area.
Good luck and I hope you feel better soon.
Infection is a big part of the painful sensation. Usually, the best treatment for this is a preventive one. Dentists usually prescribe antibiotics to prevent this from happening. If dry socket occurs, dentists will place a dry socket paste in the extraction site to help heal the site. Smoking also delays healing and increases the likelihood of dry socket by 40% within 48 hours of extraction.