Because when you reach higher level you would have more haters.If you don't make it private some stalkers might go to your profile and check all your recent activities,down vote every of your answers.Beside that it also due to privacy.Some users have few pictures inside.People tend to add you for your pictures sometimes.
To keep perverts from trying to chat with us when we are online. I know that's why I made mine private. I had guys asking me for sex chats all the time and it was very annoying. Now I know that the only people that can chat with me are my friends.
@update: A lot of guys dot it for the same reason as girls. The male xper users are not interested in cybering with unknown females. They are actually respectable guys who understand the purpose of the site.
Mine is private because I have obsessive stalkers like this one: link
That's nothing, I have a lot of GaG horror stories because people abuse the admins trust.
Good question, it wasn't that way before. Mostly women are doing this; maybe too many personal messages? Even I get a few of these come on 'let's get to know each other' messages so I can imagine the pretty women here must get dozens.
People get hassle. Making your profile private is a way of saying 'Don't give me no jive, man. Just pick up and blow on out of here.' I never got the temptation to it though, I like the idea of people looking through my photos and reading what I've written on the bio and all that, hahaha. Vanity at the price of violation, ey. THAT OLD CHESTNUT.