Question

Why do words with negative connotations have negative connotations?

Answer 1

People give words their connotations. For instance, the word aroma has a positive user-friendly feeling to it, whereas odor reminds people of something smelly. Connotation is what people think of when they first hear the word.

Answer 2

Explaining it simply, it's what people usually say to explain. Some words just have a negative meaning because they are most commonly used when something is not appealing. Such as what AlisonMo says, aroma vs odor. You don't go up to someone who stinks and say "You need to get rid of your aroma." You'd say odor.