If you only focused on the story and not the actors' chemistry, you'd get it. Hye-yeong loved Jojo ever since, even when he hasn't downloaded love alarm yet. The only flaw he had was lack of confidence, so he chose to love her secretly. On contrary to Sun-oh, he was very straight forward. Remember from season 1, Since light travels faster than sound, the thunder is heard after the lightning, Sun-oh and Jojo kissed out of curiosity and not because they were in love, though it eventually evolved to it. It all happened so fast. Meanwhile Hye-yeong had to accept it all and be patient and still continued to love Jojo in the dark. Also Hye-yeong admitted to Jojo that he's old fashioned, hence his love for antique gadgets because he always believe in the old way /tradition in loving somebody without love alarm. When Jojo finally saw all that from Hye-yeong, how can she not like him back? ❤️❤️❤️