I beg a differ. It's not that none of them found happiness. It's more like they are all insecure like Rei. I can proudly say Rei and Asahi are both happy beig together. The problem is that the insecurity is still there because Seita is pbviously still in love with Asahi and Asahi still doesn't know it so his obliviousness is what causes Rei to be insecure.
Isn't it Reí and Sei fault for leaving Asahi in the dark. He obviously wanted them 2 tell him. But they kept silent forever and that's how it ends.
When Reí picked the drunk Asahi, he went missing only to end back being with Seita. Why did he go back? And Reí just let him stay the night over there.
How can they all be happy leaving things like that between themselves without resolving it. Better they end up in a threesome I say and get it over with LOL then all of them would be happy. The Mangaka didn't gave the best of ending as if giving up on the story. Might think "Hey, let the readers decide the rest..."
so... who is the winner? It is obvious that none of them found happiness.....