I have a villager who is healthy and has full health, but for some reason, he keeps running away from his job, be it farming or research. What is the reason?

What are his dislikes? If he dislikes something like fish, crabs, or turnips, he may run away from anything to do with farming, since the occupation involves both. He may even run away if he even thinks about having to do something with whatever it is he dislikes.