Guys - Why Do You Get Approach Anxiety?

I'd much rather get a boner than start passing out. That's what happened a few weeks ago when I approached a group of S. American girls who were visiting the US. I had to end the conversation and walk away for fear of losing consciousness. Weird, I'll admit. I've met so many famous people and I never feel a reaction. Yet, some hotties...then again, these chicks looked like models. And they were friendly!