Many modern artists use their subjects and paintings to depict society’s relevant concepts. So let’s say the subject is a woman looking at herself in a mirror. The artist could envision the concept of self-image. This could be represented by a woman looking at herself at the mirror and seeing (reflected) a different person from what the observer sees. This would illustrate that we see ourselves different from what the rest see us. Whatever the ultimate framework, modern paintings serve to depict socially relevant concepts.
As a sharp contrast many traditional artists chose to rely on sublime expressions, feelings and emotions as their basis for inspiration. So if the subject is a woman smiling, the artist could also convey sadness in her eyes, restlessness by way her hands are placed on her lap etc.