The drive to retrieve is okay, as long as it is within a safe environment and under control.  

Drive: an innate, biologically determined urge to attain a goal or satisfy a need.

Prey drive, we've all heard the term and although it does come from the genetically hardwired behavior to chase a moving object; I prefer to call it just that, chase drive.  Many drives have nothing to do with prey as in animal killing.  Most of it is just the simple desire to chase something moving.  There are many different types of drive in our dogs today.  I saw this big difference when Penny was at our house.  Penny is driven by glimmers, it is becoming a problem.  She is most definitely obsessed and cannot stop unless she is physically removed from the room.  Every night when I would empty or fill the dishwasher as the sun was setting she would go off by the glimmers created.  I had to kick her out of the kitchen; obsessive drive is not something you want to fuel in any way, shape or form.

Elsa and Luke on the other hand are driven by things that they can get; lizards, crickets, balls, frisbees, etc.  Their desire to chase has not moved to the constant need to chase.  I had one of those and one is just fine with me.  Tilley, she had OCD and it was all about shadows.  She started very young; at four months of age we noticed her following butterflies.  Thinking that it was adorable we watched and watched until we noticed that it was not the butterfly that she was chasing but the shadow.  At the time I didn't realize that this was a problem; I thought she was a smart girl to find something that gave her a sure thing to chase daily.   No, it was a problem in the making.

OCD (Obsessive, compulsive disorder) is not a good thing; especially in dogs who we cannot sit down and explain to.  Her desire to chase shadows quickly moved to any shadow, anywhere.  It was not until one day that she chased the shadow of a crow into the street that I realize the dangerous implications of her problem.  So what do you do with a dog like this?  We transferred her desire to chase to objects; so that we could control the chasing.  She was given error feedback if she tried to chase a shadow outside of our yard and she learned quickly.  She became an amazing disc dog and would literally chase and retrieve anything that you threw for her.  We redirected her drive.

It may seem cute in the beginning but an obsession on something that cannot be caught is a bad one.

Obsession:  a persistent preoccupation, idea or feeling.  The act of obsessing or the state of being obsessed.

Not good in any form.  Obsession should be dealt with immediately.  Elsa cannot figure out what Penny is doing when she starts with the glimmers.  Penny cannot figure out what Elsa is doing when she is after a lizard; different, very different.  One is drive, the other obsession.  You can see Penny waiting for a glimmer when she stands transfixed, staring at the wall.  It needs to end there but it must also be redirected onto something else; something that is controllable.  Just ending it will be nearly impossible.