Why are some people incredibly insecure?

So many people I've met are very insecure with themselves. They also tend to project this onto other people around them. Sometimes, they act really mean and jealous towards others.

I can honestly say that I've never really felt that way. I feel very comfortable in my own skin. I've never truly understood this. Why can't people love themselves as they are? Why can't they realize that if they are jealous of someone else/like something about someone else etc.; that if they work hard to succeed/get help that they too can feel good about themselves? I often say to myself, wow that person is really smart and has such and such [like a degree or something]…if I work hard, I can have that too. As for beauty or psychological problems; being accepting of yourself and knowing when to get help when you need it. I wish more people would react this way instead of projecting their problems.