"Ihminen" is a countable word and we are talking about one person so nominative is used. It can be confusing when "Otsoa" is in the partitive. That's because the verb "rakastaa" always needs the partitive no matter what (it's a partitive verb, there are others too).
On the subject of partitive forms, (ihminen versus ihmistä), is that the reason that in the 'love' course we are getting so many alternate words? For example tätä vs tämä, minua vs mina, are these also based on partitives?