Jumping to conclusions
Jumping out of the dark and shouting "BOO!" at a friend is fun. Why? Because they start, maybe scream, generally act scared. Then of course, they realize that instead of a menacing ghost it's a good friend, and you both enjoy a good laugh.
Humans jump to conclusions, and they do it quickly.
If you're a fan of evolutionary biology, you might believe the tendency to jump to conclusions emerged out of a higher survival rate for those who did. If your brain says "I think I see a bear," its time is better spent calculating escape routes than contemplating, "Well what if it's just a funny-looking tree?"
If you believe god(s) made the human mind pretty much the way it is, it's reasonable to suspect she/he/it/they designed the brain to think fast and therefore live longer and more happily.
As web user experience designers, we tend to think of "experiences" as activities such as "switching tabs" or "clicking the play button."
But some experiences are moments rather than activities. Your mind sees an image and, by nature, must draw a conclusion.
If the image presented by a user interface is likely to be interpreted in a negative way ("This interface is ugly/confusing/irritating."), it doesn't matter if 500 milliseconds later everything looks great. Your interface yelled "BOO!", your user is scared, and the rest of the experience is contaminated by that negative feeling.
In some later post I'll argue that progressive JPEGs (images that initially load low-quality but then improve) are perceived with more negative emotions than baseline JPEGs (images that load top-to-bottom) because of the conclusions to which the user jumps in the first few milliseconds of the loading process.
For now I'll simply assert: user experience must be designed with an eye toward moments (even ones that last mere milliseconds) because of the human need to jump to conclusions.