Part of why Tetris is so addictive might be because of a phenomena known as the Zeigarnik Effect, Pacific Standard Magazine reports.
The Zeigarnik Effect is named for Russian psychologist Bluma Zeigarnik. Zeigarnik would instruct study participants to perform various puzzles or tasks and interrupt a handful halfway through. During her research, she found that subjects tended to remember interrupted tasks better than completed ones.
According to the publication, the effect keeps Tetris players hooked by continually presenting unfinished tasks to complete. This concept is also used in advertising, film writing and more. Check out the full story over at Pacific Standard.