For me these are the most common:
1. Rejection is always possible.
2. You're too good looking.
3. Too many friends surrouding her.
4. You’re currently occupied or unapproachable.
Most Helpful Guy
Those 4 generally nail it for me. 1. is always there, 2. I just assume you are taken or will just blow me off because "I'm not good enough for you" (leads back to 1.) 3. Always hard to approach one girl in a large group without getting glared at 4. Pretty self-explanatory.
Most Helpful Girl
I don't want to date anyone. That's the main reason.