Why did she use 'haha' in her reply?

I texted a female friend of mine (she's already attached but is pretty close to me) that I thought she was the most beautiful girl in the room in a picture, and she replied 'haha thanks! XD'. What does that mean? It seems strange to me as I wasn't telling a joke and it wasn't meant to be funny. Is she just trying to be friendly? I'm pretty sure she knows I like her.
Why did she use 'haha' in her reply?
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