may be people are not satisfied what they have our what are they. People became envy to the other because they want to experience the things they have or may be because it is they dream to be like the people where they are envy^_^
No one in the world will be ever satisfied with what they are having. They will always try to get better things in life. I would not say that this is a part of jealousy, but if someone is lacking something which his neighbor is having, then he may be feeling animosity towards that guy.
That's common in a crowded places such as tenements, row houses and the kind. People who doesn't have usually look at what their neighbors have then think that they could buy one too or have one, in the end it is more like a never ending competition between them and both would suffer from impulse buying and the budget is at stake.
Seeing someone else reaching where you are supposed to makes you jealous.
Probably yes, if you have the same goal.
Snip
Lack of self contentment. People tend to forget to thank of what they have and posses but keep asking on what they cannot achieve and attain.
Actually there is a positive side on that, it could help them to struggle harder in life and earn/have more.