Positivity is a good thing... except when it's not. Healthy positivity is about finding the good that exists despite the bad. Toxic positivity is about pretending that the bad can't or shouldn't exist, and invalidating the feelings of people who are struggling.
Saying "it could always be worse" or "everything happens for a reason" to someone who's having a hard time is not helpful. There's a time and a place for all emotions, and you probably wouldn't want to be over-the-moon happy if, for example, a loved one had just died.
I write about this issue more in these posts on Mental Health @ Home: