Smiling is often associated with weak muscle control, a natural tendency to smile, and specific illnesses. Parents need to be clear about the cause of their infant’s smiling. If it is caused by weak muscle control or natural smiling, there is usually no need to worry too much as it is a normal physiological phenomenon. As age increases, the brain gradually develops and matures, after the local muscle control ability is enhanced, this phenomenon may gradually decrease; 2, born to smile: may have a certain relationship with character development, because each baby is born with a different personality, some babies are lively and cheerful, while some babies are quiet and do not like to cry. This is a normal phenomenon in the process of growth and development of infants; 3, special diseases: some special diseases can also cause infants to smile, such as angel syndrome, this disease is a chromosomal abnormalities, the affected child can be in a state of smiling all the time. Such infants are smiling, but accompanied by symptoms such as mental retardation and abnormal motor development, and parents need to observe them carefully. If this disease is suspected, parents need to take the infant to the hospital immediately, and after detailed examination to clarify the cause, give targeted treatment.