I think sensei is just slower than others when it comes to his emotional growth. The others are already finding their love and starting their new chapter of life and it feels like you are being left behind in the race of life. I find Sensei so relatable :/ Damn when I first started this I didn't expect sensei's character to have this much depth. It really makes me care for him.
It was hinted since volume 1 that Subaru likes Megumi. It really wasn't unexpected. He was super nosy about Megumi and Tsugumi's relationship and kept giving Megumi questionable and meaningful looks.
He can fucking SCRAM tho. It's really proud of him to think he knows what is best for megumi and what he wants LOL
They make such an adorable couple ε=ε=(ノ≧∇≦)ノ