It's weird to think about it, but when I'm working on a portrait of someone, I become very familiar with all the details of a complete stranger's face and exactly how they fit together.
It makes me really appreciate how unique we all are, and how all faces have so many bits of beauty and interest that make them up.
I really urge everyone to try drawing someone else's face (or even your own) at least once.
Regardless of how happy you are with the outcome of the drawing, I think it's hard to spend that much time contemplating the construction, character, texture, and nuances of a face and not come away with a deeper appreciation of faces, and especially of that particular one.
Let me know, in the comments, if you try this, and whether you find something similar happens with you or not.