There is no definitive answer to this question, as different Shih Tzus may have different reasons for eating feces. Some dogs may simply enjoy the taste or texture of feces, while others may eat it as a way to scavenge for food or to get minerals that they are missing from their diet. Additionally, some Shih Tzus may eat feces as a way to clean their fur or as a way to deal with anxiety or boredom.