The great pain and suffering she experienced is evident and possibly achieved by the very slight downward look at the viewer and curve of her brow. Yet there is a strength and defiance in her expression enhanced by the strong lines she has used to form the brow and outline of her eyes.  

Credit: Encyclopedia Britannica

Painting eyes (in any medium including digitally) can be a challenge but also a great deal of fun. So let's zoom in and look at some close ups of eyes which have been created and you can see exactly how the artist has drawn the lines and created the patterns and colours that combine to form that look.
Credit: Live Science

They can't be seen by the naked eye but Leonardo painted a tiny LV into the right eye and some possible letters and numbers into the left eye. These can been seen with a magnifying glass. The LV is obviously his signature and the belief is that the other symbols are some code as to the identity of Mona Lisa.