WHY?

So... We've got this new cake decorater at work that I'm trying to train. I say trying because she never listens to me. I tell her what to do, how to do it, and let her loose, and she just rolls her eyes and does whatever she wants. I really do not like people like that. I had a feeling that she wouldn't last long..... Well....
It all escalated a few days ago when I asked her to set up some freezer cakes. These are really easy cakes to make, and she knows how to do them, so I let her loose. Her only restrictions were that she make 6 quarter sheet cakes, and 6 half sheet cakes. While icing the cakes, I noticed that she was having problems getting the icing down to the bottom of her bag. I told her that if she simply twisted the bag while pushing down, it would come down a lot easier. She once again rolled her eyes and ignored me. So, getting really tired of all the eye rolling, I turned to her and said. (Nicely... I PROMISE!!!) "you know, I'm just trying to teach you what I know, you really can't get mad at me for that." Well, she didn't like that very much and proceded to yell at me about it. She ended up storming out, and now she refuses to work with me.
I honestly don't get it. I was never mean to her, never rude.... Never ANYTHING except perfectly nice! What makes her think that getting mad at me is going to solve anything? Why is it that people are like that? Where does it get them? Honestly, being rude never gets you anywhere. All it does is make you less of a person in everyone's sight. I'll tell you right now, that the people you remember in 10, 20, or 50 years aren't going to be the one's that are mean to you. You're going to remember the people that helped you, or that loved you. (I'm now taking a deep breath, and releasing the anger that has built up.)