His voice was full of sadness as he talked about losing his friend.

sadness 是形容词 sad（悲伤的）加上名词后缀 -ness 构成的名词形式，表示"悲伤"这种情绪状态。后缀 -ness 通常用于将形容词转化为名词，表示该形容词所描述的性质或状态。