The creature known as man is quick to misunderstand. 
The same applies to the the pure middle-schooler Yokoyama-kun: A diligent fellow who does his infirmary duty properly. 
"You’re so nice. Thank you." The one to say that to him was his classmate Nishino-san, so Yokoyama-kun ended up thinking: ("Isn’t she only nice to me?!" ) 
He couldn’t help but be hopeful. ("Does she like me?!" )

This is the start of the misleading life between the fantasizing Yokoyama-kun and the equivocator Nishino-san.