Usually, entities simply want to survive. Unless they can find a host, they are likely to fall to dust as part of nature's normal cycle of energy composting. A fragment's only chance to extend its life is to find a person or an animal in which it can take up residence. If this happens, it will constantly draw life force from the host, resulting in symptoms of fatigue and possibly disease in the host. Apart from sucking life force, entities also often enjoy the host's emotions, desires, and sensual enjoyment.
Fragments often carry the desires of the person they came from.
For example, a fragment released after the death of someone who was used to drinking a lot of beer will keep craving beer. It is likely to float to places such as pubs, where it looks for a host that drinks beer in plenty. If the fragment can take up residence in one of the patrons, it will constantly ask for more beer, superimposing its own desires on top of its host's. The same applies to drugs in general, and to all forms of intoxication.