Out of different negative emotions, the emotion of Envy/Jealousy is most dangerous one. Just this one emotion can make you never happy. It can negate every positive thing in your life.
Let me start with a simple distinction between Envy and Jealousy. Envy is a reaction to lacking some thing that others are having it. Envy is comparison between two people. Where as Jealously is fear of losing some thing (esp in relationships). So this is between three people. But both are basically in the business of comparison.
Why this comparison in the first place?. I think this is mainly because of internal inadequacy – low self esteem. People with low self esteem tend to compare a lot. And the second reason is the mindset of scarcity. In the animal world food used to be limited so if one gets and most probably other animal will not get it. So our brains still think in the old mode. Actually we live in a world of abundance not in scarcity. If your neighbor got a luxury car you can also get if you work toward it. Actually this is very simple logic to understand but our brain fools with the old emotion.
I have noticed couple of very interesting things about comparisons
In the first place you should never compare. It is a losing game. There will be always better than you in some parameter. Even if you compare, do the overall comparison of life. Don’t just compare your wealth with other person’s wealth. Compare overall life then you will realize you are better off in other areas (may be in relationship, family life, etc)
Second thing is very interesting. In reality (most cases), the person whom you envy may not be actually enjoying what he got. Either he may be chasing some thing else or he may be comparing with some thing else ;). Below quote summarizes it very well.
“Our envy always lasts longer than the happiness of those we envy” – Heraclitus
You can instantly become happy if you know how to handle just this one emotion!!