Perhaps they are this picky because in Vietnamese culture, food is not only a matter of survival or sensory pleasure but eating is considered as external sign of good education and culture.
The personality of a person transpires through his food, with someone we just met we can only identify him through his clothing, speech or gestures, to gain a deeper understanding of his personality, habits and lifestyle, it is necessary to know about his diet.
As is the same in eastern medicine, Vietnamese people pay close attention to the balance of their cuisine to ensure good health.
Hot or cold, salty or sweet, spicy or refreshing, everything is a matter of Yin and Yang, of balance and if balance is respected, your body will not have to worry much about diseases. Therefore, the Vietnamese kitchen is considered as a small hospital for the family.