Green and red thumbs are an important feature of Board Game Arena. By giving a red thumb to a player, you signify that you dislike to play with this player. This is a subjective and personal decision, and this is why the platform won't ask you for a reason - and this is also why it shouldn't bother you to receive one from time to time. By experience, we found that this system is pretty accurate: people with a high ratio of red thumb are most of the time unpleasant players. With red thumb system, the vast majority of pleasant players of BGA can found pleasant opponents. However, we found that some (rare) players are using this system in an abusive manner. Red thumbs gave by these players has been canceled. We are going to do this "cleanup" again in the future, then there's no use for anyone to try to abuse the system. Cheers,