The reason is said to be the fact that, his mother Honjuin and his father Ieyoshi TOKUGAWA left all child cares of Iesada to Utahashi and Iesada was attached to Utahashi.
それは母・本寿院や父・徳川家慶が家定の育児を全て歌橋にまかせっきりで、家定が歌橋を慕っていたからだと言われている。