Think of the relationship between “optimism”, “pessimism” and “realism”:
generally, those words are respectively interpreted as “focusing on the good things”, “focusing on the bad things” and “ignoring (or trying to ignore) personal biases on the topic at hand”.
In a way that makes sense, the universe defines our perception on things, not the other way around.

However, let’s suppose you just had a reality check, at least as my terminally online ass knows the term as.
That means something happened to you, that forced you to realize something about yourself - be it your body, your psyche, your knowledge about anything. A realization so undeniable, that, despite your lizard brain’s psychological self-defense mechanisms’ censorship attempts, made you realize you’ve been wrong about something.

The reality check brings your mood down in the short term, and possibly pushes you to improve yourself (or, alternatively, to [concoct a workaround to the tyrannical laws of the universe]) in the long run, but… that’s not truly neutral, is it?
It may be a “bad” feeling possibly followed by a good outcome (see: cognitive dissonance), but it is never a GOOD feeling followed by a possibly bad outcome. The latter case is a confimation bias, if anything - the opposite of a reality check.

Going back to the first paragraph: if someone says “I’m not a pessimist, I’m a realist” you may conflate that person for an pessimist, but not an optimist.

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