A combination of fear of rejection and stage fright, aka “Love Shyness”.
Approach Anxiety aka " Super Freeze". When you see that hot girl and get stuck inside your head you are actually entering a strong freeze response. For many they enter a state of dissociation,( stuck inside your head), and become mentally frozen. The reason this happens is that the hot girl is seen as a threat to their fear of rejection or abandonment. The result is procrastination to approach the girl or brainstorming if they should even talk to her. For some people who have a "super freeze" response exhibit social isolation and heavy daydreaming. You have to understand that social isolation is very unhealthy and can create a "cult" inside your mind. Extreme examples of social isolation are the herbivore boys of Japan that stay in their rooms all day and watch anime for 15 hours a day. In extreme cases, some guys decide to go MGTOW and avoiding women altogether. You have to realize that you can be brave and succeed in the sexual marketplace. The antidote is to work on self love and self compassion and realizing that "you are enough" and "assume attraction".
Very few men don’t feel at least a little anxiety in the moment between seeing a cute girl and thinking over how he’ll introduce himself. (We call those men lacking any approach anxiety, “psychopaths”. Or, “blacks”.) Obviously, some men will be more anxious than others. For these sufferers, a mental or behavioral short circuit that bypasses their anxiety modules can mean the difference between intractable incel and endless samplings from the poon poon platter.