why do people get jealous?

Jealous
–adjective
1. feeling resentment against someone because of that person's rivalry, success, or advantages (often fol. by of): He was jealous of his rich brother.
2. feeling resentment because of another's success, advantage, etc. (often fol. by of): He was jealous of his brother's wealth.
3. characterized by or proceeding from suspicious fears or envious resentment: a jealous rage; jealous intrigues.
4. inclined to or troubled by suspicions or fears of rivalry, unfaithfulness, etc., as in love or aims: a jealous husband.
5. solicitous or vigilant in maintaining or guarding something: The American people are jealous of their freedom.
6. Bible. intolerant of unfaithfulness or rivalry: The Lord is a jealous God.

source:http://dictionary.reference.com/search?q=jealous  ¶ 11:37 PM