Being a portrait artist I study how a person's face changes at different times, both subtly and unsubtly.

These are the reasons that I see behind those changes:

1. The most obvious reason, the lighting.
2. The atmosphere of the place. Meaning, one looks different in a foreign place than when he is at home.
3. How the person is feeling.
4. What the person is wearing.
5. The present company of other people and whether the person is comfortable with them or not.
5. What the person ate at least an hour before.

Indeed, we never look the same as we used to at any given time. And perhaps that is why we call every painting and photograph "a moment frozen in time."