Do premature infants tend to have a different outlook on life than full term infants?

I have two nephews under 24 months. One was born premature and tiny, less than two pounds. He has good physical skills but he is so serious all the time. It takes a lot of work just to get him to smile or show any humor. He’s not angry or displaying any other signs of bad behavior. His parents work with him all the time to get any smile or laughs out of him. My other nephew is as happy as could be. I’m just curious if the premature infant’s early experiences stay with him? He went through a lot of rough experiences. Any thoughts?