Why is she angry that I left without saying goodbye?

MobySansei
I went to keep this girl company when she was by herself. Then I left as soon as her other friends arrived.

I thought she wasn't paying much attention to me so I just left without saying goodbye. I got a message from her a few minutes later and she complained how I left without saying goodbye and that she was mad.

I like her a lot... and she admitted to a mutual friend of our's that I am her bf. HOWEVER, we are not officially in a relationship.
Why is she angry that I left without saying goodbye?
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