I remember reading a book from the YA SciFi/Fantasy section of the local library in the mid 80s, around the time I was discovering Asimov, Heinlein, McCaffrey, McKillip, Norton. Our young teenaged male hero "just graduated" or otherwise is joining a rocket ship, either as a medical officer or as the junior officer on a medical ship. Everyone on the ship is human, but the Captain is mad at him because "the Fleet(?)/Patrol(?) doesn't allow pets" yet our hero has a fuzzy little creature that lives on his shoulder - an alien emotional symbiote or some such rather than a pet. The rocket ship traverses alien planets, exploring or going to the assistance of a number of places or adventures, and our hero is always at the center of the story, learning and growing up and becoming part of the crew. The one subplot planet I particularly remember is the aliens who ask for help because a disease is rendering these "low tech" intelligent aliens back to a non-intelligent animal state. The medical crew finds an unusual bacteria in the aliens and starts killing it to try to cure the disease, but that speeds up the problem. Turns out the bacteria is the intelligence, and the "aliens" are just the host - killing the bacteria was killing the ones asking for help. The solution, as I recall, was to have the hero's symbiote replicate and be the host for the bacteria. Thanks for any leads!