It's been said that the eyes are the window to the soul. This applies to photography in a major way.
When capturing any sort of portrait or image we want to depict the subject as best we can and using a Catchlight is a crucial part to this. Yes Lens choice, surroundings etc are also big parts of the equation but eyes without a catch appear dead and vacant.
There are numerous ways to ensure there is a catch in your subjects eyes, find a window, use the sun or a bright object. The most common ways are to use a reflector or a strobe. Most flash units also have a bounce card which is a little white card you pull up and it will create a nice catch in the subjects eyes.
With some lighting styles in the studio is hard to get the catchlight directly from your key light, e.g. if you are loop or Rembrandt lighting for example. You can use a strobe on low power just to create the catch you want.
There is always post processing, adding a catch after the image is taken in Photoshop or Portrait pro. This can produce amazing results as the above image shows. BUT be cautious you don't overcook the eyes when doing so. I see endless images each day where a photographer missed the catchlight on set and added one in post only to give the models eyes a overglow. If thats the effect you are after then great but if its a natural portrait then you will need to turn it down. Agencies wont accept images with this sort of editing..
Dont overburn the eyes, it simply looks out of place in a portrait.
When I need to edit eyes in post, I always create the setting I like then dial it back 25%, look away, then look back. I find 90% of the time this looks natural.
Of course getting it straight out of camera is best, you can't go wrong.
Play about with it different lights also creates a different catch, a strip creates a great effect with sun glasses on and a ring or octo is amazing for close up eye makeup shots.