My reason is that graffiti is a form of expression. You can sorta see the feelings of a person in their graffiti, such as anger. People usually use blues, reds, whites, and blacks to express anger. But for happiness, people use yellows, oranges, green, and light colors. Therefore, graffiti is expression of emotion and thoughts. Also, so people use it to mark territory or to get their point and view across to others.
(What do you think the person painting this felt?)