Most Helpful Girl
It's all a game... a challenge everyone seems to want what they cannot have and when they can have something they don't always want it or appreciate it.
The "side chicks" probably appreciate you more or to you feel like they are purely because they don't fully have you and they feel like by doing this they will possibly 'get you' I can almost guarantee that it would not stay that way after a offical relationship was formed. Once that happens they won, game over.
This is also not to say that the people or in your case females don't have deep feelings, they just don't feel that they have to constantly show them because they have already got you. Just like once guys start dating a woman they stop doing all the little things they did to get them in the first place.1