I remember the last time I was jealous, it was because I felt inferior and I was scared of losing what I had.
But from what I've seen, envy's different. The last time I was envious, I simply didn't have what it was I coveted from the other person, and I decided to work my ass off to get it.
I think that's the qualitative difference between jealousy and envy. Jealousy's destructive, because it gnaws inside and tells us we're worth less. Envy is the desire for more, to want more, to have more, to be more.
Channelled positively, I think envy could be a great driving force for change. When we see others having something special we want for ourselves, we'll work harder to have it. Certainly, envy has been in the past channelled negatively to more destructive ends, but as I said, these are tools which can push us towards good, so long as we are aware of what we're doing. (To be honest, most anything we do should be accompanied with a good dose of self-awareness. But that's another post for another day.)
I'm envious of people who do some things so smoothly, like it's no effort. I want to be like them. If I work hard enough? Perhaps I shall be! I'm talking about a kind of envy that hangs out with hope, in order to motivate us towards reaching for ever more, ever higher, ever better goals.
Those who do not envy, well, great for them, too. But there are some of us who aren't content with what we have, and we can either get self-destructive in jealousy, or we can take our envy into our hands and use it to drive us out of the sad state which breeds our malcontent.