This entire time he has been searching for the feeling of security and safety that Sumire has brought him. Only when he first started dating Sumire did he know what he had been missing. That missing hole in his heart.
Something was always missing, no matter how much he sang or painted his emotions. Sumire filled that hole subconsciously. Being with her made him feel so safe.
His most peaceful moments were those mornings and nights where they could lay in bed, stay in each other's arms, and just cuddle. The moments where he felt the most safe and secure.
At one point in his life, he started to use the strength he obtained to protect others. For so long, he has been protecting other people. But he finally had someone who would protect him, someone who would watch over him, keep him safe.
A partner who would fight to the ends of the earth for him made him feel safe and loved.
For everything Sumire thanks him for, Yuhi could double it.