I think that he "love" starts at different times for both characters. For Rak, it starts at episode #5, when Diao helped smooth out the misunderstanding between Rak and his little brother Rit. There is a tender scene when Rak says "khop-kun na" (thank you) to a sleeping Diao. I think that is when Rak started officially catching feelings.
As for Diao, I think it was episode #12, when Rak stood up for Diao at work, and put his job on the line to help Diao. Even then, I still think Diao has yet to come to terms with his own sexual orientation, but he knew he felt something deep.
I hope and pray the show gets a season 2, there are so many things that can be explored. Diao's coming out story, The shenanigans that happen as they try to maintain a relationship on the sneak tip, we could meet Daisy's boyfriend "Pom", we could also meet Dao and Diao's parents. I am dying to see their mother! So many possibilities. Thanks for keeping the discussions going JPNY01! Your comments have kept the discussions going on here! lol.