Why is it difficult to tell the ones we care about most?

My best friend and I went out to breakfast this morning. She knows something is bothering me. I just couldn't do it, I couldn't tell her. I do not do well with the face to face thing. I am really good with online chat or e-mail. Why is it so difficult to tell someone you care about very much something that is such a big part of your life? It's not like she is going to change her opinion about me. I know she is still going to be my best friend and lunch buddy.