I think it reflects on how they know that everyone has ups and downs and how every dark side has a light side.
If we were to only feel joy all the time, we wouldn't even know it because nothing would be different from one another.
There would be nothing to distinguish a difference, so that being said, for example:
when you have a bad day but come home to your significant other, your mood changes and you can then count on the good times to get you through the bad ones.
I think that's what encourages them and gets them through their issues.
Hope this helps:)
