When Asuka was four (in the year 2005) her mother was in the hospital - her mother was insane. She'd gone insane while synchcronizing with an Evangelion - Evangelion Unit-02, to be exact. Her mother worked for Gehirn and, unbeknownst to Asuka, had gone insane trying to synch with Evangelion Unit-02. She often talked to a doll, one that beared resemblance to Rei Ayanami (whether this is a reason that Asuka hates Rei or not is unknown to me). Asuka's father never paid much attention to his daughter - he cared more for his work. Asuka was assigned as the new pilot to Evangelion Unit-02 and was named the Second Child. She was to start training. She ran to go tell her mother of this great accomplishment...and when she entered her mother's room in the hospital, saw her mother hanging from the ceiling with the doll's head on the ground, torn off. This scarred Asuka forever, and from then on she vowed never to cry. And she didn't. Not when she watched her mother's grave, not even when she lost for the first time. Her past was tragic and defines what she is in the TV series - a girl with a lot of power and pride, who uses it in such a way to make some people dislike her. Asuka lives to be told "I love you", because she was never told those three words after her mother's death.
Another picture of Young Asuka