"you fall in love with your bestfriend's lover,
Will you betray your bestfriend just for the person you love?"

->If you alone fall in love w/ your bestfriend's lover, then you are not betraying your bestfriend, unless the so called "lover" falls for you too. Otherwise, just keep your loves w/in you and it may passed as times goes by. Like MJ says, it maybe just an infatuation that the "lover" is just too lovable for you to felt attracted ;)