Dr. Eggman invented a serum that will make the injected person act upon their deepest desire. He believed that Shadow's was to kill Sonic. Boy, how wrong he was... [eventual Sonadow]

Love is when you care for someone more than yourself, and you would do anything for them to be happy, alive, and well, no matter what the cost. He wasn't supposed to fall in love with his rival. He wasn't supposed to let things spiral out of control. He wasn't supposed to let the other get away with everything...but he does, because he loves him, no matter what the cost. Sonadow.