They are not so defensive as they seem. They just know when you are not really into them, and you're just desperate for a piece of meat.
When i do aproaches at a club this is what happens all the time:
- If my friends tell me to go aproach a girl, or if i just saw her in the moment and go there, im going there just randomly, and im really not interested in meeting the chick, i just tought she was ok and went there. Aproaching a girl when you're not really interested in meeting her takes off 90% of your game and confidence, so this is when i usually get rejected, or just small talk.
- When i check out a girl for a while, see who's she with etc, or if we exchange eye contact and so on, if i really want to meet her i NEVER get rejected at starters. I may not bang the girl in the end, but i'll have a nice sexy conversation with her, prolly dance with her, get her # if i dont get lucky right that night.