Saliva remains on the spoon, the saliva contains the enzyme amylase, which breaks down carbohydrates. If saliva gets into common food, then the process of decomposition of nutrients will begin. Microorganisms, viruses, etc. and so on can also be transmitted through saliva.
Because people, even if they are your relatives, may be disgusted with the thought that your saliva gets into the dish along with a spoon. Some people forbid drinking from the throat of the bottle for the same reason.
This can only be explained by cockroaches in the head of the one who does not let you do this. All my life I have been eating salad from a common plate with my spoon, I bread soup from a saucepan sometimes, when I’m too lazy to get a separate plate. I have not infected anyone with anything, and I have never spoiled food.