Why does my female friend tease me and look at my lips a lot?

So this girl and I have been friends for well over a year. She already knows I like her. But she picks on me a lot. She teases other people too, but I get picked on way more than anyone. Particularly she makes fun of how I'm single and lonely, and how I can't respond to a flirting girl and how I can't ask a girl out. We got in a misunderstanding which resulted in me in telling her why I like her, and she typed in a bunch of hearts to me and said she was flattered. Then when I saw her the next day she ran up behind me and gave me a big hug and said "You're so sweet" to me like 20 times, and rested her head on me. But whenever I'm in class she just stares at me for a long time, and it always seems to be at my lips, and she did that today, and even playfully mocked me once. Why is she like this?
Why does my female friend tease me and look at my lips a lot?
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