It's because we believe we can do anything and we are in awe of a girl whom we like. I was like that.
It's completely normal to be shy! Why wouldn't they be? Girls are hard work, we expect a lot so of course we can scare guys off sometimes.They're trying to ask a girl out , they want to impress her and make sure they leave the right impression. If that isn't nerve-wracking what is?
He is afraid that if the girl says no, then shes going to tell EVERYBODY that someone asked her out, and she said no.
They don't want to get rejected by a girl.