excitement

if girls see a guy they like a very much, they can become very excited 

and when girls become very excited, they tend to run away 

it's probably an old mammalian instinct to run away when excited, 

because excitement could mean that there is a predator nearby 

for girls to keep cool and not run away when they see a guy they like very much, could be very difficult for girls 

these types of instincts are difficult to prepare and train for, 

but your instincts could possibly be used for your own advantage 

pretend it's a game, gamify what's difficult in life and stress will likely decrease