This means to focus on the negative aspects of a situation and downplay, or not see, the positive aspects. In a way it has three aspects. Our choice of focus in the world, what we dwell on with our outer senses. Also, what we create with our inner/spiritual senses, i.e. the mental-pictures we create, we choose to imagine negative things and negative circumstances. Actually, we don’t so much ‘choose’ (although that is possible), all these scenarios and fantasies and daydreams and bull**** come up spontaneously, often triggered by something in the physical world, but not always. Thirdly, there is our reasoning. We (semi-consciously) choose to (mis)reason by dwelling on negative data, assumptions, evidence etc.
So a person could be analyzing a date, how it went, what the food was like but mainly the interaction, and it was mixed, good and bad, but while there, they’re focusing on any hint that it’s going badly, the stuttering due to nerves at the start, dropping some food into their lap, but not the affectionate hand squeeze at the end, the smile, the long conversation about movies and discovering they both like the same genre.
So in real-time, present continuous, shall we say, while it was happening, they chose to perceive the negative, and hours later are sitting alone at home torturing themselves, remembering each negative event (e.g. the food drop) and not the positive (e.g. the movie conversation) — and then this focus (with filter) goes on to create images on the mental screen, of past (negative) memories of social encounters that went badly, or imagining possible future social interactions going badly. Perhaps creating a daydream of the date telling their friends how badly it went and about the food drop, but not creating the daydream of the possibility of them telling their friends it was great and how they realised they have the same taste in movies.
The fact is, you have a choice, of what you focus on in the world, the pictures you create in your mind and the way you think about things. You can choose whatever you want, starkly, between sanity and insanity. It’s your choice. You make your bed and you lie on it.