To sadden is to make someone feel sad, or to become sad. The death of a beloved dog is likely to sadden the whole family.
You can say that it saddens your friends when they see vulnerable people bullied or discriminated against. You can also say that your friends sadden you when they tell you about pollution that's causing animals to go extinct. Before the verb sadden came along in the 17th century, the verb meaning "to make sad" was sade, from an Old English root.