One of the hardest things, when you have a Shih Tzu, is to understand and interpret their thoughts. There are times when you do not understand a thing from their look, or you will try to use human interpretation for things that come from a dog.
If there is a thing you have to learn when you have a Shih Tzu, it is that they have different thoughts and interpretations than humans. These things you can only understand when knowing the psychology of Shih Tzus.
Shih Tzus live the moment, so the things they do will last only as long as they are doing it. One example is that when you come home after work, your Shih Tzu greets you as if they have not seen you in ages.
It may be moping all day, but when they see you are happy. Or when you stop their playtime, but their thoughts will change fast. To hold grudges is not their nature.
Another thing is when your Shih Tzu had a trauma in the past, they remember the sadness and the pain that trauma brought to them.
So your Shih Tzu will not hold any resentment towards you. They do not have the long-term memory that we have.