[The hand she just tried to brush away is stubbornly back, encircling her slim neck loosely with the help of his other hand; as Saito crouches down closer to her height.]

I was born with a brain disorder, in which my damaged pituitary gland doesn't secrete oxytocin properly. To put it in simple words even you can understand, I can't experience things like love and intimacy the same way most people do. On the flip side, my brain rewards me with large amounts of dopamine and endorphins when I take a life.

[There's not a single shadow of dissatisfaction or regret in his tone during the explanation, as though he's talking about something trivial, such as the weather. In fact, there's even a short laugh from him before he sums up the explanation in even fewer words.]

In case you can't connect the dots even now, I'll spell it out for you. I only feel "happiness" when I'm killing something.