Yes, it is quite complex, it all depends at which level the particular guy is.
In general, usualy nobody stops us, it is only our own mind, our (limited) believes that don't allow us to move forward.
Guy A sees a pretty girl, and his mind goes something like: She is so beautiful and hot, it would be great if she were my GF and we had sex. Maybe we can live together, maybe it could be love for the whole life, maybe I could marry her. But I have to approach first, I have to talk to her. I will probably mess up, I won't have much to say. I have to find out what kind of guy she is looking for and then be that guy. What if I'll make a fool out of myself, what if she realizes that I am not the guy she is looking for. I should definitely be much better person to match her beauty. Chances are high that she will reject me. She wouldn't go out with me, I'll get my hopes up and then I'll be rejected. I don't even have balls to ask her out because I know that she will definitely reject me, there is no way she will go out with me, she won't even talk to me. If she does, I don't know how to be sexy, I don't know how to touch and kiss, and I will most likely mess up during sex...
... and he goes on and on, 10,000 thoughts like this run through his mind each second... He then shuts down, he gets all anxious, all afraid, and he just won't approach... he walks away, he never talks to that girl. He goes home and analyzes, and analyzes, and creates weird theories, reads many books, tries to understand all different girls, differences between blonds and brunettes, and the more he reads the more complicated it is, and the more he doesn't understand... But it's all in his head, there is really nothing to understand...
Guy B sees the same girl, his mind says: Wow, she's hot, lets go talk to her, maybe there can be much more than talk... If it becomes awkward I can simply walk away. And he goes and talks to her. His mindset is totally different, he has no limited believes, his self esteem is much higher, he doesn't put himself down... he trusts himself, he believes that he is a great guy, he doesn't need any weird theories, he simply goes and talks to another human being... It's also in his head, he didn't need to build any obstacles, he didn't need any theories to talk to that girl...
And the pretty girl? She sees both guys, both seem equally attractive and sexy. She knows that both have dicks. But Guy A walks away. What's with him? she's asking. He is just not interested in me, that's ok. Maybe he's got GF, maybe he's shy, maybe he's all in his mind, who knows. But the Guy B is approaching, he's got balls to approach, he is not afraid to talk to girls. Let's see if there can be some fun with this guy...
So some of the negative believes:
* I can't do it
* I can't talk to her, I don't know what to say
* I will mess up
* I am not smart enough, I am not sexy enough, I am not attractive enough....
* I am not what she's looking for, I have to be ( should be ) the guy she is looking for
* I have to seduce her, I have to be great seducer with sexy walks/talks, the sex have to be great
* It takes lots of balls to approach girls. The approach has to be great and clever
* She will not go out with me, she is too hot and she is looking for better guy than me. She must be looking for A, B and C in a guy, and I am not like that
* She will reject me, I can't take any rejections, I was already rejected several times. If this girl reject me I will be devastated
* What do I have to do to be her BF? What does she want? What is she looking for in a guy? I will do everything to become her BF!
Some positive believes:
* She is cute/silly/sexy girl, let's go talk to her
* Talking to girls is easy, it's just fun
* There is always something to say, even silly things are good enough
* There is nothing to mess up, it's just fun. Maybe I don't know everything and I am not the best in sex but let's find out/learn together
* I am a great guy, I am smart enough. I am attractive/sexy enough
* There is no approach, there is nothing great or smart about approaching. I just go and talk to that girl, right there. L0et's see what she's about, let's find out what kind of person she is
* I don't need to seduce anyone. Seduction is very easy, the better we click the easier it is
* If there are great vibes we will definitely go out; she will want to go out with me because I am interesting/attractive guy
* There is no rejection. I'll never find out if I don't try. I'm already doing my best and if things get awkward I can always walk away with a smile. Life goes on, there is plenty of girls. Next!
* She needs to do A, B and C first before she becomes my GF, I won't make it so easy for her. I am looking for D, E, F in a girl, let's find out if she meets it. I am who I am, I know that the relationship has to be managed but I am not really going to change myself because of a girl