When they are outside Headquarters, Sadness tells Joy:
"Without you, Riley can't be happy. We got to get you back up there."
But why would she help in bringing happiness? She should've said something like "Let's just stay here, don't bother trying"
Personal motivations of characters are not very well defined in this movie. Sadness shouldn't even be able to grasp a concept of a happy Riley, so she can't contribute to it.
Didn't you notice how Sadness was constantly trying to influence the Core Memories and others?
Don't forget, Riley and her family recently moved from Minnesota to San Francisco, and she was extremely homesick and missed her old home and friends, and I suppose Sadness reflected that in her own behaviour. As Joy finds out, her friends and family came to support her and cheer her up BECAUSE of Sadness, and that's really what Riley needed when she was at her lowest ebb and running away. Sadness stopped her at the last second, and made her return to her parents for support.
That's what I got from it, anyway.
Each of the traits also have Riley's conscious as part of their collective thinking.
Sadness alone might have said "don't bother" but as a collective, we're aware of all our thoughts.
Sadness knows how Joy works. Sometimes the joy is even greater having suffered from sadness.
I think you're looking at it in too linear a manner.