Why do I feel sad when I see a beautiful girl?
That is self rejection.
Since you are an introvert and never had a female friend so when you look at a beautiful girl, you want to talk to her.
But due to the above mentioned reasons, you convinced yourself that why would she talk to you? She will be having lots of friends and so on.
More you look at her, more you want to talk to her and more is the number of (self) rejection.