Why has music changed from the 70s and 80s when music was upbeat and happy to where it was in the 90s and today where most music is sad and depressing?

Everything in this world changes, like for ex everybody back than had old dress fashions and today we have different ones things change now a days and just because some music is sad and depressing doesn't mean ALL is, there are some like pharrell William's happy song brings joy and Lil uzi vert's XO Tour Life brings sadness and depression, and music has changed because people want it to change, say for ex you're watching a tv show and it will not come with more seasons or updates and u watch it for 20-30 years straight, wouldn't u want to watch something else to give it a try and maybe like that one, yes u would because your taste change and so does the world, there is no such thing as a person who doesn't change anything like hair, clothes, attitude, personality, those things can be changed but still the point is the world and people in it have their own tastes and do new things.

Wanna know why
everything changes no matter what 
things get worse an worse 
once you think it gets better guess what it doesnt 

Ireally don't know the answer