Girls, Why is this girl and I naturally hit it off but she has a boyfriend?

She gets overly happy to see me, but at the same time she doesn't know me that well but she and I really enjoy chatting to each other. She and I both agreed we didn't get a chance to chat much yesterday. But today she and I spoke for 30min straight. She complimented that my hair looks nice. I could tell she and I both enjoyed our conversation together. I really like her but found out she has a boyfriend.
Girls, Why is this girl and I naturally hit it off but she has a boyfriend?
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