It was easy, at first. Find a place to hide. The place happens to be a baby. Use the simple humans, let them care for you. Wait fifteen years, and you can return.
Then, the love.
First, the love of a human mother for a child who despises her. Watch her struggle on, single mother, what does she see in me? Never had this love from my demon mother. See the scars blossom on her arms where she saves me from hitting my head on glass. She would die for me. Why?
Love. Plain and simple; insidious. Gradually the human world, which I never blinked at before, begins to wear me down. I find myself looking forward to telling her what I've done in school; looking forward to her taking me out, like her child, to eat in a pizza place. I find myself distressed when she is unhappy. I have never allowed anyone, any friend or lover, this level of control over me; now she, the woman in whose womb I sought shelter, has snared me.
Still, I have not fallen. Wait a human's lifetime; protect her, and nothing else. I have no qualms about pretending to be the perfect son; I have no qualms about murdering behind her back to protect her and her world. She will not be harmed; she will live out the remainder of her life with a son who loves her dearly, and die a natural death. This is the right order of things; this is what I owe her for life. Steal to save her? Die? In a heartbeat.
The second I saw you I knew. Why do you think I left so fast? Don't have time to be arrested, indeed. I could already feel my ties to Hiei and Gouki slackening when I saw you. Call it an animal's instincts. I knew--I knew you would understand.
And you did, didn't you? Only you did more than that. You rewarded my trust with something more; life. Your life for mine. In a heartbeat.
Everything changes, now, again. I become insanely curious about you. I pester Koenma with questions about you; I raid his files when he won't answer. And I fall hard for you, for the impossibility of your task, the desperation of your life. I have only one thought. Help.
So here I am. Famous demon thief, legend and nightmare of thousands, working to defend the human world. And not just because of her anymore, either. Because it matters to you. Because you've chosen to. I even sucker Hiei into it.
I know I will die eventually for it. I've betrayed my own kind, and betrayers do not live. I'm ready for that. It's the least I can do to atone for my sins.
I've always kept a step removed from you until now. I do not threaten Kuwabara's position as your best friend; nor do I threaten Keiko's position as your beloved. I find your devotion to both of them astounding and touching; I know you are similarly devoted to me, and until now I have contented myself with that.
But lately I find I cannot.
You changed my world, Yusuke. Even loving Shiori, my heart was hardened to the plight of all those I did not consider mine. You changed that; you were my downfall. It is as Hiei always says; my fondness for humans will be my undoing, someday. And I have found that I cannot bear for that to happen until you know.
Know that I fell, Yusuke, for you.