Why don't people follow me back on Twitter?

[Sarah]

I had to repost this because it was in the wrong category.
Anyways...
Why don't people follow me back on Twitter?
I got Twitter a few days ago because most people have twitter now instead of Facebook. A majority of the people I've followed havnt followed me back. I know these people, their people I know from school work that I have talked too & get along with. Isn't it a little rude that they wouldn't follow back considering we know each other? I've followed 20 and only about 10 have followed back. I have 13 tweets so far and I have a profile picture. Why won't people follow me back? It kind of hurts my feelings. I even made a announcement on Facebook to follow me. So I don't see what I'm doing wrong