Getting Passed the Let"s Just Be Friends Stage She"s Put Up
You're probably wondering why she still glows when she sees you like she did before and why you can't seem to get beyond the "lets be just friends" stage she's put up for the both of you. The answer to the first question is that she still most likely loves you as much as she did before. The answer to the latter question is that she's trying to block that love from you and herself. She doesn't want to get let down again.
There are a few reasons why she might have left in the first place even though she was still in love with you. She might have been feeling a little unhappy; like things weren't going well or moving fast enough and she was tired of waiting. She might have been afraid that things were moving too fast for her and she wasn't ready to jump into a commitment with someone. She might have just been so complacent with life with you she might have stopped recognizing the signs of actually being in love so she figured it was best if she stopped leading you on. Any of these and other possibilities exists as for why she may have left you.
What you have to do if you're in this sort of situation is to help her see that she still loves you. You're also going to need to work at building her trust again so she's not afraid to let you see her true feelings about you.
The reason she wants to keep you around as a friend is because she recognizes that you play an important part in her life. You just have to work to show her how important and what part you're supposed to play... as her boyfriend and companion rather than just a friend.