Yashiro is in love with someone he knows he'll never be able to have. It's not so much that Kageyama called him out saying he's all alone, alot of you guys think that's why he broke down. I think he broke down because Kageyama, for lack of better words, friend zoned him.
It's a painful feeling wanting to push them away because you know you're going to get hurt, because you're not good enough- but not being able to push them away. He saw all the faults in Kageyama AND STILL wasn't good enough. It's a pain like no other..... just my two cents.
Tadayoedo Shizumazu, Saredo Naki Mo Sezu