If two balls are placed beside the wall, forget about moving them. If two balls run into each other in a passage, forget about moving them. Thus, it will only be possible to finish the levels pushing the balls, moving round in circles and trying not to block any, something that will become rather difficult as we progress in the game.
Furthermore, to increase its addictiveness, Sokoban saves the movements that we have used to complete a level, so another challenge that we can have another go at each level trying to decrease the amount of movements.
Sokoban appeared in Japan in 1980 and became popular very quickly due to the simplicity of its rules and how addictive it is. Its name means “Warehouse manager” in Japanese and it really represents the idea behind the game, because the game consists in organizing everything using the shortest path.