The first reason we don't take enough photos is purely down to inspiration. If you're not loving the scene then you wont feel compelled to shoot a lot of images. Fair enough. The second reason is storage. We worry about filling up our cards and having sufficient hard disk space to store them when we get home. Buy a bigger card and stop being cheap is my answer. That's not a genuine reason when 8 gig cards cost around $50.
The third reason, and the big one, is perfectionism.
I was inspired by an article written by Amy Harrison about the perils of writers who set their bar too high. Writers block is one symptom of reaching for the stars when the moon would have done nicely. At the other end of the process writers can be trapped in a cycle of improvement and re-editing that prevents them from being satisfied with the story. They never finish the pieces in fear that it won't be good enough.
The lesson for photographers is very similar. I have noticed this potential in my own work many times.
I recall years ago being in the Greek Islands to shoot a brochure for GAP Adventures and being completely overwhelmed by the beauty of the place. Island after island of stunning charm and all the colours you see in the postcards. It's as lovely as people say it is. So were my images going to be good enough, to do it justice. I struggled one day to capture images, hesitating on the shutter and walking away from the scene in search of a better subject. Finally I realised that I was passing on 'average' shots in the search for the brilliant one.
I made a conscious decision to go ahead and shoot my images even if they weren't brilliant, they weren't the best ever photos of the Greek Islands and weren't my own best work either. Two things happened. I started collecting a bunch of adequate shots. That's important on a commissioned shoot as not every shot has to be the cover of a brochure. All the other shots of chairs, and coffee and laneways are useful to the brochure designers even if they're just 'adequate'.
The other thing that happened is that my shots started getting better. And that's an important lesson to learn when shooting images. Taking more photos doesn't simply mean taking the same photo again and again. There's a serial process that happens as you shoot, as you make subtle adjustments from one frame to the next. You search for those variations in your composition and keep shooting as you go.
Taking more photos means exploring more ideas.
This is especially typical of how I shoot still life photos. I don't just get in close and snap away. I have learned that too often I frame too tight, so now my standard technique is to keep shooting the scene as I pull back away from the subject. With each frame I take in more elements, more detail and more context. I get better photos as a result. The same can work for you when shooting portraits if your tendency is to get too tight. Start with your natural shot, then keep going by either pulling the lens wider or stepping back away.
You can think of this as 'bracketing the composition' if you like. Or you perhaps the analogy of drawing a draft copy is more to your liking? Whatever the language that works for you, use it and start taking more photos.
It'll make you a better photographer.