To put it crudely, we must force ourselves to keep on paying attention. Ranking the pictures focuses our attention on our favorites. It also focuses our attention on ourselves, which is in fact our favorite topic. Me, me, me. It sounds crude, doesn't it? But if the "Me Factor", as I will call it, is operating against the art rather than working with it, our love affair with museums won't last very long.
This a very effective technique. We deliberately activate this factor during Reverse Brainstorm sessions, when participants are asked to "invest" into problems. The attention, and as a consequence, the quality of input is much higher than during regular brainstorming. People tend to take their investment responsibilities very seriously, even when it is done with "funny money". We further strengthen their motivation later, by giving them several more opportunities to make their choice count, either through problem analysis or problem solving.
on p. 54 he also mentions the biggest scarcity of all: "our time and attention are scarce."
another thing that is really scarce, especially in the group dynamics context, is trust.