To change things up. To shake their surroundings. In other words, they are bored. To make someone else happy. To provide laughter to a friend. To horrify someone. In retaliation! Anything basically. There are so many different reasons.
I colored my hair blonde since I was 12 because I wanted to stay a blonde. Now, 8 years later, I like my natural hair color. So I changed it over a year to its true color.
A girl I used to work with bought contact lens that changed her eye color to blue or green. She had brown eyes. It always made me happy. I would walk into work wondering what color her eyes would be like that day. The bright green of my ex boyfriend or the bright blue of Superman in the latest Superman movie.
Some women are not happy with their breast size, so they change it. Yes, I think it juvenille. However, it happens.