Why does my girlfriend lead on other guys?

Ok my I think my girlfriend leads on another guy…why is that…and he(that guy) wrote a song about her she wants to hear it why? like in our city we have a sub place that allows locate musicans to play there,,,my gfs brother plays there a lot and we went…that guy was there to but before we left she asked that guy to play hos song for her…with me there I have wrote her a lot of songs…but that guy likes her for a long time and he is scared of me bc well…im never the nicest person to him…well as we lwft she sadid I should be happy bc she sat beside me and not him and me you know instead of haveing me on one side and him on the other (we have or I have had problems with this she used to do that a lot, it made me feel like I had to compeate for her attenion I dont like doing that) so she didnt do it last night…anyway also my girlfriend has a flirtly personalty and I dont like her flirting with other guys of course…she says she doesnt mean to and she loves me…what do I do with that to the whole her flirting and not meaning to and me not likeing it…give me some advice over all this please