Sadness is literally the very definition of sorrow and gloom. She is the most sensitive and melancholic out of all the emotions, as she felt genuinely hurt by Joy's abandonment because "Riley needs to be happy", which made poor Sadness feel she was truly useless and unneeded. Sadness also complains a lot, as seen when she keeps saying she's too sad to walk or do anything, much to Joy's frustration. Sadness is also smarter, has more common sense, and is more pragmatic than the others. 