It is normal to feel a little sad or worried when you see others all around you making it big in their respective fields in life and you just seem to be stagnant. It is such a saddening feeling that gradually develops but if it is not controlled, you begin too desirous of what they have and jealousy comes in crowning the whole envy thing which is really unhealthy..
People turn to be envious of others out of sheer jealousy to see others making it and not them. This is very vommon in humans nowadays as jealousy has been the character of the day that friends became so jealous of their friends that they even end up harming them. Envy and jealousy are two things that floe together and where you find one, the other's presence is not a doubt.
Also, so many people have been pushed to be envious of others because of pressure imposed by their families, loved ones and society on them. Many people out there who were content and leading content lives have turned to be envious as a result of pressure imposed on them by the society. It is common to see how parents lash out on their cgildren comparing them to their other mates who are very successful. These only mounts pressure on such children and only promotes envying in them.