What causes pathological jealousy? Taking the meaning of the word pathological to mean maladaptive and unhealthy. 

What is/are the aetiology of pathological jealousy?

What conditions are associated with features of pathological jealousy?

Discussion can concern neurological or psychological problems or both.
I think there are many determinants of jealousy. The very basic one is evolutionary explanation. For example, jealousy was necessary for male to extend his genes. If he wasn't, the another male could "take over" his female partner.
Another (more interesting for me) explanation is psychoanalytic one. You can read more for example in Bowlby's books, but also more classic ones. As "more classic" I mean these works which talk about Oedipus complex or other phenomenon which influence our approach (e.g.projection) to other people in future.

1) Buss,Larsen,Westen,Semmelroth - Sex Differences in Jealousy: Evolution, Physiology, and Psychology

2) White, Gregory L.; Mullen, Paul E. (1989) Jealousy: Theory, research, and clinical strategies.

3) Pines, Aronson (2006), Antecedents, correlates, and consequences of sexual jealousy

4) Bowlby (1953) Some pathological processes set in train by early mother-child separation