Envy is an empty sin which dwells on what we have not, while resenting those who have. We are covetous of goods, but we are envious of people. And envious of them because they possess some thing or quality we do not. In this envy not only leaves us dissatisfied, but it also drives a wedge between us and our brothers and sisters.
We envy people's possessions, their qualities, their bodies (the covers of fitness magazines are a testament to this). We even envy people their spiritual gifts and relationship with God. Instead of practicing thankfulness for our own gifts, we wish we could pray like Louise or preach like Frank.
Envy is different from admiration. We need to admire others, to have heroes - people to emulate. But admiration is a form of love and is therefore self-forgetful. Admiration does not lead us to envy because it has nothing to do with us. We are simply appreciating another. When we start comparing ourselves, our emptiness has crept in and we are envying.
Envy is the empty vice because it it rooted in our own emptiness and loss. The cure to envy can be found in the Beatitudes, specifically, "Blessed are those who mourn, for they shall be comforted." (Matthew 5:4)
Just as anger is an indication that some boundary has been crossed, envy is an indication that some loss has not been grieved. My daughter, who has lost all of her grandparents, recently told me how she finds it hard to see grandparents spending time with their grandchildren because that can't happen for her. As it stands I think that she is actually grieving her loss - feeling sad about something she has not. But one can see how easily this can turn into envy, resentment of the other because of what they enjoy which I cannot. And one can see how it is driven from emptiness and can drive a wedge between us and others.
In addition to grieving our losses, we need also to practice the Christian virtue of self-acceptance - being at peace with who we are, how we are, and what we have (both goods and gifts). Many will be afraid of this because it sounds like complacency and we mustn't be complacent. (Why I mustn't be complacent I cannot articulate but I know urgently that I mustn't - this should be a clue to me that something is off here.)
If we struggle with this notion of self-acceptance perhaps we could benefit from considering how God sees us. He seems to have no trouble accepting us in spite of our sins and emptiness, and yet has plans an intentions for us to be more, to continue to become. Self-acceptance is really just accepting the truth about ourselves - reality as it is. If we are trying to drive to San Francisco and are currently in New York, it will be of no use to wish we were in St. Louis, and resent the people who are already in St. Louis. None of that will get us to the Golden Gate (double entendre intended.)