(Another bloody good book from Friday)
In essence, jealousy and envy both stem from an inadequate childhood. A child grew up without proper parenting (which is extremely hard to come by) have the intense desire to be liked. This can manifest in terms of such person constantly wanting to be in the center of attention, constantly wanting to show the world that he or she has "made it", is "happy" and is worthy of desire. However, as Friday showed in the book, when such person encounter a competitor that can pierce his/her "ego balloon", the person will grow increasingly jealous and envious and will find a way to sabotage the competitor.
This can come in many ways. For instance, for guys, it's very common for such person to want "bigger, better and shiner" things, such as bigger house, nicer car or more pretty looking trophy girlfriend/wife. For girls, they have the innate desire to be the most beautiful person in a room (and if she isn't, she'll invest heavily on her appearance, or even move to an area where there are less beautiful people). They also have innate desire to constantly be liked by others, they want to create an illusion that they're saught after by other people, they're popular and "don't even need love from others because they have plenty". They create narriatives for themselves and often are the only one that don't realizes how lonely they truly are.
Friday has noted that true self esteem doesn't come from the illusion of desire, but rather a healthy childhood (which Friday has repeatedly repeat in her books that she didn't have, since she has never met her own father). A child grow up with a proper parenting doesn't need superficial narratives to prove they're loved, for that they already know they're special, they're loved, from their parents the moment they were born.
Perhaps one of the biggest takeaway from the book is that it's not healthy to engage in people who have big egos - you can never win. They'll always have the intense desire to feel their superiority (which masks their inner inferiority) and when you do demonstrate your own superiority in a objective way, they'll grow ever increasingly jealous and have the inner desire to sabotage you. "Envy is such a killer", as Friday said.