Truth behind the lie
He'd never admit it, but when he'd let Jenova into his mind, let her soothe away his sanity, he hadn't believed any of her lies.
He hadn't listened to the whispers of caring, the promise of affection, the claim that she was his son and she loved him. Plenty of people had claimed to love him because they wanted something from him, whether money, power, status, or just lust. He knew better than to think that just because he carried her genes she cared about him. Hojo was his father, after all, yet the man had never seen him as anything more than an experiment, a step on the road to the ultimate creation.
It hadn't even been the cleansing rage that she offered him that had finally swayed him, convinced him to allow her into his thoughts, her alien mind twisting his feelings and ideas until they aligned with hers as well as she could make them. He knew that once anger burned itself out he'd be left with less than he'd started with, guilt and shame mixing with the horror at having lost his control and the fear that he'd gone too far this time. Anger was like all other emotions – it was messy, uncontrolled, and ultimately, deceptive.
No, he'd submitted himself to the alien's will because he could see the truth behind the lie. He knew that she would discard all that made him who he was and leave behind nothing more than a tool, a being that didn't think or feel anything but what she wanted him to think and feel, and that once her tool had served its purpose she would discard him without a second thought.
He'd been a tool all his life, and he'd hated it. No one had ever offered to make him enjoy being used before. Sephiroth had given up his free will for a taste of illusory happiness, because he'd never thought that he might be given anything better.