Everyone is talking about how Suna ended up single, but he made his feelings pretty clear. I've just finished the anime and I believe Suna could have a number of reasons why he wants to be single.
For starters he admitted that he hasn't ever met anyone he's meshed with, he's also said that he's enjoying his life right now as girls are a bit too much for him to deal with, so to me it looks like Suna is just Demisexual and is waiting for the right person who's personality grips him enough to date them. He also told Takeo that it takes a long time for people to fall in love, even though we see Takeo and Rinko almost fall straight away.
I believe that Suna likes the girl who gave him anonymous chocolates throughout the years, but it's going to take him time to know her better before he can reciprocate her feelings. After all, we didn't exactly see him turn her down like he did with all the other girls, and he did enjoy his time with her. I believe he loves Takeo as a brother, because he's a good person and he genuinely enjoys his time with him. I could be wrong and maybe Suna does have some kind of mixed feelings about him, but I don't really believe it myself.
I think Suna is happy just being Suna, and he's just taking life one day at a time, and I think we should just enjoy the story for what it was instead of what you think it should have been.
^_^ Have a nice day!