My younger sister ran away when she was only 12. Although she does have a number of mental disabilities, she left because she wanted attention and felt as though she was not getting the attention she thought she deserved. She was crying for help and was punishing our mother for not doing what she wanted.
Teens runaway or may attempt to for a number of reasons, I myself never did however I can see why some want to or do.
They may want to be on their own or they may feel they are not understood, that no one cares, unwanted or want attention so they lash out by running away in hopes they'll get the attention they're craving. Once they're away, and they see their parent(s) or guardian(s), worrying, crying or looking for them the attention they're craving, they see that someone does care.
Every teen is not the same by any means, but the main reason to running away is simple: they want change. Rather they want to be on their own or simply want to see who cares.