Why does she look at me? Does she miss me?

There was a girl I was going to ask out but I found out she kissed another boy. That got me angry and I started arguing with her because I thought me and her had a spark but she didn't notice so we argued. I told her I was going to ask her out but that guy stopped it. After that I went into depression for 3 days and then I got over her, Im happy and enjoying life but at night I think of her. Why do I? In school (I'm 15) I hang with my group and I sometimes look over to her group to see if she's ok and I always catch her looking at me with a depressed look like she's looking at me with a little of a sad face, a little sad face, not a lot!. Her friend looks at me too. Why is that? She doesn't hang with the guy she kissed anymore. Why is that? My main question is why does she look at me? She has that boy she kisses she can be free, I've moved on.