Is it her laugh? Confidence? Or a sexy body?

Of course, there has to be an initial attraction but none of those things will compel a man to make the first move. Men have been taught (by women) that approaching them outside a party or a bar or some other social event is unwelcome violation and an inappropriate form of harassment. Most men are good guys who want to do the right thing and the message from women is "the right thing" is to leave them alone.

The only thing that changes this is when a woman essentially makes the first move by signalling that she's open to being approached - prolonged eye contact, friendly smile, flirting, etc.