I was thinking on what form was used, but seeing that the point was giving nifuji the little a confusing confession, using love there works to explain why he feels the way he has.
Because loving someone can vary in meaning, and if someone has profound fondness of another person, you could describe your feelings as love, without them being romantic necessarily, and it comes as a shock that Kou could feel that way for him, but with the following words he's left for a loss of words because, yes, she could also mean "I love being with you and all that you've given me because it's been that valuable, as a friend". But it also makes him think through his own feelings, thinking what would've happened if she actually did meant it romantically.