Man, fuck Shin.
It's understandable not wanting to see either of your friends get hurt, but he's not even making a distinction between a friendly, but intense, competition and a hate-fueled fight. His inability to relate to Yuu's and Shougo's desire for a sense of belonging, as well as their competitive spirits has been a real drag the last couple chapters, and its especially frustrating seeing him throw Yuu into a depression because of his reactions now.
Yuu gave Shougo what he wanted, showing him respect in the process. Likewise, I think Yuu desired to test himself again and express himself with his friend, "talking" through their strength. I think Shougo in his way was showing Yuu respect too.
Shin just spoils the nobility, intimacy, friendship, etc of the situation with his whining and misinterpretations. And to cap things off he makes Yuu feel abandoned, misunderstood, and alone for giving his friend what he wanted and being himself in the process.