How do you understand difference between 'all things of that kind' and 'all the things referred to by a noun'?
We use a/an with a singular noun to say something about all things of that kind: A man needs friends. (= All men need friends) A dog likes to eat meat. (= All dogs like to eat meat)
We use the definite article to say something about all the things referred to by a noun: The wolf is not really a dangerous animal (= Wolves are not really dangerous animals)