Jealousy when seen in a small child, whose family has just grown to include a new baby, is easily identifiable as an insecurity. The child is worried that there won’t be enough love and attention to go around, and that they will receive less.
In adults it can be harder to recognize, but it still has the same roots. Someone else has, or is perceived to have, something the jealous person wants.
Identifying jealousy in your own or someone else’s writing can help deal with it.
Knowing from where the behaviour stems makes coping with it more possible.