Why does my friend like me when she has "cooler" friends? What's so special about me?

[How]

Ever since we first met 2 years ago she was one of those extremely gorgeous girls but she was super nice and friendly to me, always stared at me, always complimented me, said hi in the halls, invited me to work in her groups, smiled at me, always interested in what I had to say, etc.

Eventually she gave me her number and we'd text and talk on Facebook all day and night, before, during and after school...We became close like that, she opened up a lot to me sometimes (she's naturally pretty reserved)...but in person I would hardly talk to her, ignore her sometimes...because I was dealing with some issues...nothing against her I think she's great.

We're seniors now and she's still so nice to me. She has a lot of friends...All of them are those super confident, exuberant, talkative, outgoing type. They all seem like a lot of fun, and like I said my friend is extremely pretty...so people are obsessed with her, always inviting her to hang out and do things. I swear it seems like one of her friends is like in love with her. All she talks about is my friend. And on twitter a lot of her posts are about her hanging out with my friend...But she's pretty cool...kinda "fake friendly" though.

But anyways why does my friend still like me? I'm really quiet usually, she's seen me at my worst, we've had some pretty awkward moments together... But in class she's always staring and smiling (and she hardly ever smiles, people always try to make her smile) at me. One time we were at this place...I was sitting with my friend somewhere, then she came over with her group of friends and smiled at me, then kept staring at me. In the halls she's always so excited to see me, reaches out to give me a hug. Invites me to do things with her. She's really accepting of me. Whenever I talk she's always listening and nodding, smiling. When I used to ignore her a lot and treat her pretty sh*tty, she'd forgive me right away when I apologized. Two of her friends act like they don't even like me (including the one I mentioned), but I don't even think she notices...I don't get why she likes me. We aren't even close friends really...
we're both girls btw