We tend to devote most of the attention we pay toward our own photography to our portfolio shots; after all, those are the images that we show off to the world, that carry our brand, and that instill pride from our accomplishments. However, we can learn just as much (if not more) from the images that didn't make the cut.
Coming to you from James Popsys, this great video follows him as he dissects some of his images from a recent trip and how he feels he could have improved them. Personally, I tend to learn much more from seeing where I went wrong in my failures than trying to understand where I went right in my successes, and I highly recommend sitting down with your own images and doing this yourself. If you have trouble critiquing your own work, grab a photographer buddy whose opinion you trust. When I was in music school, we used to do what we called a "post-mortem" after every recital, and despite the histrionic term for the exercise, it was tremendously helpful to sit in a room with my peers and hear various perspectives on how my music was constructed and performed. It's absolutely worth taking time to do a quick review of your work after each shoot.
I agree. I toss images after looking at them and thinking, "it's rejected because I......" It might be that I missed focus or cut off a foot or didn't get a very good background, etc, etc. But there's a reason for my rejection.