An expressive portrait or self-portrait is a style of painting, it's not about the expression on the person's face. Whether the person is happy or sad, smiling or frowning, is irrelevant. How the paint has been applied is what's relevant.
Compare the two self-portraits shown in the photo. They're clearly both paintings of a face, and even if the photo caption didn't tell you they were by the same painter you'd probably have thought it was the same person depicted. What is clearly very different is the style in which each is painted.
The portrait on the left is painted in a realism style, which mimics what we typically think we see. The colors used for the skin are "real", the paint has been blended to create a smooth finish on the skin. The portrait on the right uses colors you don't expect for skin tones and the brushmarks are strongly evident.
Color and mark-making have been used expressively in this painting, to move the portrait away from something that's a likeness of a person. You may not like the final result, but it's got an impact that the realistic portrait doesn't have. Imagine the painting on the right was titled "Sea Sick" -- how do you feel about the colors then?
An expressive painting style uses paint to do things that you can do only with paint. Some artists take it further than others, as you can see in this Expressionism photo gallery.
