Eyes are the key element in a portrait. It is worth to pay attention to them while taking a picture.
When looking at a picture, eyes are first attracted by the brightest point, which usually must be in the EYES. Eyes REFLECT sources of light (flash, reflectors) which create bright dots in the eyes (even cat eyes with soft boxes). The more dots, the more glamorous (look at Georges Hurrel and Clarence S.Bull here). An eye without reflection looks dead, which is not really flattering. Therefore, if there in one point that must be IN FOCUS, it is the eye.
Eyes with REFLECTION of light.
Eyes in FOCUS.