My cousin used to be that way and I had no explanation for it. I just stopped texting her back because I was so disapponted in her.
I also have friends who are very demanding, but very unavailable themselves. I stopped investing time and effort to "fix" things.
It is like the other posters have already said: Only she knows her motives. If you feel like someone doesn't have the best intentions when contacting you, it really isn't someone you want to be around.