Kim Jongin was the youngest of the 9 Kim brothers. After their parents died soon after Jongin's birth D.O, being the oldest, took the lead and took charge. Because of the age difference between D.O and Jongin, D.O never really acknowledged Jongin and only took care of his basic needs.

Even though D.O knew it wasn't Jongin's fault that his parents died he always felt a pain when looking at him causing another reason he didn't bother to be close to him. When their mother was giving birth to Jongin she died in the process and their dad died soon after of depression and shock. 

Kim Jongin never really thought of D.O as a brother. D.O was only their for him when he was younger just to take care of his basic needs. Other then that there was never any interactions. Because of that D.O felt like a stranger. A good looking one. D.O was always such a wonder and mystery to Jongin. Though he resented the man just a little because they were brothers and he never treated him like one, he mostly had a big crush on him. Jongin had a crush on a stranger who just happened to be the man who lived with him and was his brother.