Women are attracted to what they cannot have, and they no longer want what they can have.
To explain, 'bad boys' represent the gritty, testosterone filled man's man they are attracted to, these bad boys do not pay attention to their women and that makes them want them even more. Why is that?
When a man doesn't pay attention to woman, woman subconsciously try to get the attention of the man. Once a man gets to the point of doting on a woman they cease being her lover and subconsciously she becomes turned off. Now marriage does work IF you inject love and romance into the relationship. Without that there are only two things that will keep a woman with a man. Money or Bad Boy mentality.
This is why many woman cannot get away from abusive boyfriends, because they crave that negative attention, and if you think i'm just blowing hot air there are many studies you could google on woman and abusive relationships.
You are her friend not her lover for two reasons:
1. You are too caring towards her and not what you SHOULD be, a independant but romantic man's man.
2. You went in to help her as a friend, that was probably your first thought too. That it would branch out into a real relationship, men all over the world get into the trap of wishing their friendship was something more. Thats why the phrase "Let's just be friends" is a Coffin Nail to a man's ears.
You must treat her right, buy her flowers etc. never cheat but never get involved in small talk, and baby her as little as possible, this is what her girlfriends are for.