Chihuahuas rely heavily on their sense of smell to perceive the world, and they may react differently to different people based on the scents they detect. They can also use facial recognition to communicate with each other and may associate certain features with past threats or negative experiences. Other factors that can influence a chihuahua's reaction to someone include past socialization, past trauma or abuse, and a desire to protect their territory or family. While it's important not to judge people solely based on a chihuahua's reaction, prolonged aversion to someone may be a sign of potential issues. Training and positive reinforcement can help a chihuahua learn to feel more comfortable around new people.