Power asymmetry and also group pressure. There is still plenty of tacit pressure to share despite the "optional" caveat. Someone who says directly "I don't like this, no thank you" even in very deferential words will garner some degree of negativity, either from the manager or the group. Same for someone who shares too much and winds up sobbing, or someone who shares a vulnerability inconsequential in comparison to what others share.
twelve40|1 year ago