Here, a girl is described in terms of what she used to be and what she has become. She once enjoyed being alone, but now she is afraid of loneliness. Her smile used to come easily, and now it is fake. Even music, which she loved to listen to, has become something she lives inside, as if it's her only shelter.
The speaker seems to be watching from outside, almost like a friend who understands her. The casual "You know why, uh?" feels like a quiet challenge, asking the reader to notice what is really going on. The answer is that her silence is a kind of sacrifice. She hides her feelings to keep other people happy and to make their problems disappear, loving them more than herself. In the end she gives away her own happiness.
The mood is sad and tender, with a sense of resignation. The short, plain lines mirror her silence: nothing dramatic, just a slow fading of the person she used to be.