Well, on the upside a feisty baby is usually a good eater. When they're little, everything else is a potential predator and they'll either flee, hide or defend themselves. The first option isn't available to them so it's either ball up and hope you (big scary monster thing) lose interest in them and move on or make themselves look like big tough guys and scare you away. As they get older/bigger they'll settle down and be more relaxed.