My six year old just got his teeth cleaned and the dentist said he had an abcsess in his tooth and they needed to extract it that it couldn't be treated with antibiotics? It is a healthy tooth, no cavities and the fact that it was'nt painful bothers me . Where did the infection come from and why can't they treat it without pulling the tooth?

Gum boils (abscesses) can drain. If the bump on the gum goes up and down day to day it's because the pus is draining out of it. When this happens there is usually no pain as there is no pressure building up.
The tooth may have been injured a year or so ago and has since died. The reason the dentist may want to remove the tooth is so the infection doesn't cause any problems with the adult tooth that is developing under the abscessed tooth.
If the tooth that is being removed is one that shouldn't fall out until your son is 8-12 yrs old you should have a space maintainer placed so he doesn't loose the position for the adult tooth that is to erupt.

It's very hard to do a full root canal on a baby tooth (ive tried and failed) due to the anatomy of the roots, so usually they just get extracted... An abscess is not painful because its draining but if it stops draining it can get VERY serious and very painful so please get it taken care of right away

I agree with Flossie and Amir- that it is best to have the tooth removed to avoid further problems. The dentist is suggesting removing the tooth because since it's a primary tooth, he'll be losing it anyway. Depending on how soon the tooth would normally fall out on its own, the space maintainer is the best option to avoid the adjacent teeth shifting as the permanent teeth grow in.