What causes tooth abscess? ?

I can only speak from my own experience but I had a deep cavity in one of my molars that went untreated and eventually led to an abscess (infection). I had to get a root canal on the tooth to save it. So I guess decay can lead to abscess. 

Infections cause this stuff

A Periapical Abscess is caused by bacteria getting into the gums and/or through the tooth enamel for various reasons in addition to already having weak enamel from poor nutrition or medicinal drug use by mother or child in its early years.  These include poor dental hygiene, a cracked tooth or a loose filling etc.  The pulp inside the root canal can die and infection and inflammation then sets in. They seldom if ever go away without treatment. 

Bacteria causing an infection around the root of the tooth.

Bits of food that get wedged in or under gums and your body tries to get rid of it. 

Essentially it's a spot/pimple but in the mouth. Which is basically what an abscess is anyway. A cyst or abscess is a pocket of pus that develops when a foreign body makes its way somewhere it shouldn't. Just a really really bad spot/pimple. Though, it isn't always pus.