Is memory random?

Why do we have the memories that we have?  What makes something "stick" in our mind?  Why do we remember some seemingly random, inconsequential things and forget what seem to be monumental events?

My memories from childhood seem to be associated with years in school.  I suppost those were the markers of my life then.  I remember a few scenes from kindergarten, a few from first grade, one or two in second grade.  But I remember vitually nothing about 3rd grade--nothing about the class, the teacher, my friends, events.  Fourth grade is hazy, but from 5th grade on, things become a lot clearer.  Is turning 11 significant developmentally?  Does the brain change then?  Do we become more aware of our surroundings?

Going back to pre-school years, I have memories that I can't believe are really mine. I think they must be stories I've been told enough times that I remember them.  But, there are "images."  For instance, I clearly remember my father knocking through a wall connecting the new addition of the house to the old.  I remember playing in the foundation of that same addition, but I would have been very young at the time.

I'm surprised sometimes at the physical nature of my memories. I remember how I felt more than I remember what I thought. Recently I heard a young man interviewed. He had been in a car accident when he was 10 or so and was left with his legs paralyzed. He says he remembers nothing of the accident, yet sometimes will have feelings in his body of flying through the air, hitting his head, etc.  His body remembers the sensations of the accident even if he doesn't recall them consciously.  I sometimes feel the same way. I can "feel" something that happened or with a strong emotional memory my body will recreate the physical sensations of the time.  I feel it in my bones.