Retrospectives, and my lesson learned: think positive!

Yesterday, I had retrospectives with my team. Every 2 weeks we do retrospectives in order to learn something from ourselves. While running a meeting like this, I am asking for several things. The mood must be positive, and all the negatives things shall will be called less positive things. Finger pointing is also not allowed, unless we all want to create a tension that needs to be resolved by the team.
During the retrospective, the normal question sequence is something like: what went OK in the last 2 weeks, what went not ok, etc.
While the team talks, I am in front of a whiteboard drawing a mindmap with all the ideas, problems, and lessons learned. A retrospective lasts 1 hour if it is regular and the time is held every 2 weeks.
To conclude a retrospective, I usually ask for something that is not common.
Yesterday, I asked: In turn say a positive quality with an adjective for every colleague. After this question, my colleagues' were like a deer in the headlights  eyes were like the eyes of a cow watching a train. I confess, they struggled very much in finding a lot to find a positive adjective for their mates.
My lesson learned is "think positive." I realized that thinking positive is quite uncommon nowadays.