Portrait photographers and wildlife photographers are both crazy about catch-lights, but if the two groups were to fight it out, I have to say that wildlife photographers are even crazier about catch-lights than portrait photographers.
Catch-lights are specular highlights in the eyes of an animal or a person. Look around you for something shiny and round…. a metal lamp, a mouse, anything. Notice how they always have a reflected white highlight on them? That's a specular highlight. Specular highlights are a visual cue to the brain that your eyes see something round. Your brain subconsciously knows that the highlight would be larger and more rectangular if the item weren't spherical.
Photographers have the challenge of making 3d objects look good on a 2d screen or print. Anything we can do to trick the brain into feeling like there is depth in the picture will help us to make the viewer feel sucked into the photo. A specular highlight will help photographers do exactly that. By including catch-lights, the subject is given a dimensional shape which feels more realistic and natural.
So how does this apply to your photography? You need to get in the habit of looking for catch lights when you shoot. A portrait photographer may ask a model to tilt her head one way or the other so her eyes catch the light just right. Both portrait and wildlife photographers will often use a flash for the sole purpose of creating a pleasing catch-light in the eye of a subject who is already well lit.
Is it trivial to think of such a minor detail as a catch light when out photographing? Possibly, but if I've learned anything about photography in the last few years, it is that the more little details you get right, the more powerful and captivating the image becomes. Watch for the catch-lights.