Quick question for Muslims; girls and women especially...?

I've asked this question to Muslim girls before and the responses vary.

The "hijab" , the head covering and all that most or a lot of Muslim women wear; I understand the main reason given for wearing that mainly is modesty; but those of you that chose not to wear it or cover up when you go out; my question(s) is why? Why do you personally choose not to wear it if your religion mandates it?

And whatever your reasons; are you not technically sinning against your god by not being covered up when you're in public? If so aren't you willingly choosing to sin then?

I'm not trying to start an argument or anything just curious, cause I have a Facebook friend in Africa that's a Muslim and I've seen her pics and see that she doesn't always wear the covering. When I asked her she basically said she wears it when she wants to. I'm in college and have a female Muslim friend who also doesn't wear the hijab; she dresses normal like any other girl; still modest in my opinion with jeans a nice shirt and she likes to wear short boots. When I asked her about it she got a little bent out of shape cause she doesn't like answering that question.

Again not starting an argument just curious as to some Muslim girls rationale for not wearing the covering when they go out. And for the Muslim guys reading this question, what are your feelings about the Muslim girls that dont wear the hijab?