For sure it is a big Mitzva to always focus on positivism and expect goodness. But if you don’t have a streak of belief in the possibility of Middas HaDin, then you can run into grave trouble, Chas V'shalom. You simply can’t accept a situation that is contrary to your desires, and when evil happens, Chas V'shalom, then you have an overreaction.
People who aren’t serious about the reality of Gehinom which is very difficult to escape (and 1 minute of Gehinom is worse than the entire Nazi Holocaust) suffer worse, Rachana Litzlan. A person who believes in Gehinom can withstand pain better, and he is somewhat alleviated since you lose tons of Gehinom when you have pain down here. כִי אָּדָּם אֵין צַדִ יק בָּאָּרֶץ אֲשֶר יַעֲשֶה טוֹב וְלֹא יֶחֱטָּא (No one is perfect).
