a. The clothing tells me that he is boring, he is serious, and he didn't want the focus of the portrait to be on his clothes.
b. The background tells us that he is a pretty simple, normal, nice guy and he wants the focus to be on him and not the picture.
c. The artist wants the viewer to feel serious. Because the portrait is plain and simple.
d. His name is Frederick, he is really rich, and he is serious because he has so much money. He cares about hair and money.