Dating

Why am I so stuck on this girl/can't stop thinking about her?

Ok so there's this girl that i met in one of my classes this semester. The first day i saw her i thought she was ok, but for some reason as time went on i started to think about her more and more. She started off sitting a few seats away from me for the first 2 weeks. After that she started sitting next to me everyday. I'm pretty quiet in class so i never really talked to anyone. She started looking at me a lot especially when the professor said something funny. Soon after this became a normal thing and we started talking just a bit but not much. Outside the classroom whenever she saw me she would just say "hey" and continue with her business. There was one time in class when she teased me a bit but i didn't take much from it. Whenever i ask her a question or say anything to her she usually looks me right in the eyes and smiles or laughs a little. She also likes to play with her hair and put on lip gloss in class all the time.

Not long after i was pretty confident that i liked her at least a little, but now i'm sitting here thinking about her a lot and i want to tell her what i think of her... heck i was even stupid enough to write her an anonymous letter... She's read it but no response either she isn't interested or wants to know who wrote it i guess. Anyways I've been thinking about her all day and was even thinking of knocking on her door and just telling her today but i chickened out.
if you want to read the letter it's here: https://www.girlsaskguys.com/girls-behavior/q2740115-did-i-screw-up-was-it-a-mistake-to-write-her-a-letter

Any advice?
Why am I so stuck on this girl/can't stop thinking about her?
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