Why does paint smell long after paint job was finished?

Well it really depends on the type of paint. See there are paints that are low VOC (low Volatile Organic Compounds) these are the ingredients that cause paint to smell. More oil based paints tend to off gas "stink" while the VOC compounds are curing, thus the long lasting smell. Low VOC pains tend not to smell so bad because of the formulation of the paint, less compounds that need to cure during the drying process.