When people wear black a lot, why do people automatically label them as goths, emos, satanatic worshipers, etc

I wear black a lot, but I'm not of those, why do people label others so quickly just because of what they wear. I mean just because a person wears a shirt that has "Jesus Died for my Sins," on it, doesn't mean he believe in God or believes that Jesus died for his sins. I don't get the stereotyping.