"The man whom I love is serious but funny."
Translation:Mies, jota rakastan, on vakava mutta hauska.
The logic is the same that causes the English relative pronoun to assume the form "whom" instead of "who". This whole sentence isn't a single clause because there's a relative clause within it, and the verb in that relative clause can't target something outside of its clause. The verb "rakastan" can therefore only act upon the relative pronoun "jota", which makes it an object. Since "rakastaa" is a partitive verb, it requires partitive case for its object.