I believe that you think too much, but at the same time, I like to analyze my own and other people's behavior.
First, think about the fundamental level of the contact between males and females. I know to some people it's horrible to say that, but we're in a science forum, so I'm not too afraid of the reactions: humans are nothing more but animals. We share the same goal as dogs,
monkeys, and any living being. We want to reproduce to make the race evolve. If that weren't our goal, then we wouldn't be here today.
Humans just tend to find more subtle ways to this, in order to respect each other and
then try to make the world a good place for everyone. If you don't eat your neighbor's
children, then he will not eat yours and so on.
I might be a bit far from the subject, my point was to say that talking to a girl/boy is something that comes "naturally". The nervousness, imho, comes from two things. The
first is the society we live in which made classes: popular girls/boys, nerds, etc.
The second is, still imho, based on some instinct: if I go talk to the girl, and my profile doesn't fit her needs, then I will be rejected. Rejection means no mating and no mating means no kids and thus the end of my "family". As I said above, it is a need brought by evolution. All the rest is details.
If we just reproduced and then died straight away, this would stop there. But here,
even after you have kids, you keep living for years. Thus comes the need to avoid pain.
Living in a group reduces the probability that you will be in pain. The reason we live
in a couple of 2 humans is something that I can't really explain though.
Ok, I don't know if I said anything useful or interesting about the subject. If not,
feel free to tell me and I'll delete my post ;)
Edit: Ok, I just realized that this thread was in the Relationships forum, when I thought it was in some part of the Biology forum. So I think you might not like my answer at all :P