I think jealousy can be stemmed from a variety of different sources. Sometimes it has to do with a person's past; if one has been cheated on or lied to by a significant other in the past they tend to be more jealous in fear that it will happen again. Other times it may be because they think of straying from a relationship so they fear their sig other may have similar thoughts. And then there's always people with self esteem issues. If you can't appreciate yourself it's pretty hard to imagine that anyone else would appreciate you. I'm sure there are many other causes and that they vary from person to person. I'm not a very jealous person, so this is pretty well the only insight I can offer.