Trust me on this, if people say they need time alone, then they need breathing space. You can only be part of someone's world if they let you, and a lot of people (me included) have a habit of fighting back when people try to enter against their will. It's scary when you tell people not to approach you and they approach anyway. As if you have zero control over what people do to you. As if you have no right for privacy.
She most likely would do it because she needs breathing space, and if she doesn't get it she WILL get pissed/moody/angry. xD