Jealousy is comparison. And we have been taught to compare, we have been conditioned to compare, always compare. Somebody else has a better house, somebody else has a more beautiful body, somebody else has more money, somebody else has a more charismatic personality. Compare, go on comparing yourself with everybody else you pass by, and great jealousy will be the outcome; it is the by-product of the conditioning for comparison.
Mentally, physically and emotionally we are the same.
We each have the potential to good and bad and to be overcome by disturbing emotions such as anger, fear, hatred, suspicion and greed. These emotions can be the cause of many problems.
On the other hand if you cultivate loving kindness, compassion and concern for others, there will be no room for anger, hatred and jealousy.
Hatred, jealousy and excessive attachment cause suffering and agitation. I feel compassion can help us overcome these disturbances and let us return to a calm state of mind. Compassion is not just being kind to your friend. That involves attachment because it is based on expectation. Compassion is when you do something good without any expectations - based on realizing that "the other person is also just like me".
Jealousy in romance is like salt in food. A little can enhance the savor, but too much can spoil the pleasure and, under certain circumstances, can be life-threatening.