The main cause is not being happy about yourself.
Well it is close to that. Jealousy is one of the 7 sins and no one can ever stop themselves to commit a sin ever since it started by the first two people in this world which is Eve and Adam.
Jealousy is the sin we commit especially when we are seeing people having a lot of things we do want to have for ourselves and that is not that because you are not happy about yourself but you are not contented to what you have. We need to be contented to what we have otherwise we will be starting to see other peoples that we will be jealous about.