Why do I have these feelings for her?

This girl and I have been friends for a year and a half now and best friends for like a year or so. I'm not sure what it is but sometimes I get erflies when I'm around her. She has a boyfriend, they are both serious and we are all friends so theres no rough edges or anything. We constantly laugh when we're around each other and the other night she said I was her "best friend" although I only see her maybe once or twice a month if that. We've been through a lot and she's always been there for me as have I and I think we trust each other to an extent. I guess I'm just wondering how I get these feelings of always wanting to be around her and wanting to talk to her? Second when a girl tells someone that they are a "best friend" are they really telling the truth? I've never put her relationship in jeopardy and I just want her to be happy even if that is just continuing to be friends. Thank you!