What is that picture you wish you took?

I just stumbled upon this website: "http://www.unphotographable.com/"
It’s a photographer’s blog-ish-thing that is a collection of all the pictures he didn’t take using language.
This made me think of a couple things–first, is there a picture that you wish you took but didn’t? Why not?
why do we (as a society) take so many "meaningless" pictures (I’m thinking of a lot of what comes up on facebook…teenage girls somewhere doing nothing, pictures of feet arranged in a circle, etc.), while we are often too scared/forget to take the truly important ones?
I usually don’t take many pictures myself, because I feel like when I have a camera it’s too easy for me to stay behind the lens and not experience what I’m documenting, which normally leads me to rely on others’ pictures or memories or writing it all down or something. Is it possible to fully experience something while still documenting it or recreating it in an artistic way?
Also, what pictures have you taken that you treasure? Are they different from what we find on Facebook and the like?