Why do people say we have to go through bad times sometimes to get to the good?

So what they’re saying is bad is good?
Sometimes my dad says struggling and going through tough times can be a good thing because it make you a stronger person and you actually work to get to what you wanna do or need to do.
My sister said if life was sunshine and rainbows all the time, how would we know the difference between bad and good and we HAVE to go though bad times sometimes and we can’t especially things to good and comfortable ALL THE TIME.
Wow.....that’s harsh.